A   TREATISE  OR  ELECTRO-CHEMISTRY 
EDITED  BY  BERTRAM  BLOUNT,  F.I.C.,  ETC. 


THE    ELECTRO-METALLURGY  OF  STEEL 


A  TREATISE  OF  ELECTRO-^HEMISTP.y\ 
EDITED  BY  BERTRAM  BLG4JNTYF.LCU ETC*      .    . 


THE    ELECTRO-METALLURGY 
OF   STEEL 


BY 

C.   C.   GOW 

Assoc.R.S.M.,  M.I.M.M.,  B.Sc.  (ENG.) 


WITH  A  PREFACE  BY  DONALD  F.  CAMPBELL 

M.A.,  A.R.S.M.,  M.I. MM.,  M.I.E.E. 


NEW  YORK 
D.   VAN   NOSTRAND  COMPANY 

EIGHT  WARREN  STREET 
1922 


%?*,-,•'.-.„»;,;       .      *      -'"" 

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PRINTED    IN    GREAT    BRITAIN    BY    THE    ABERDEEN    UNIVERSITY    PRESS    LIMITED 


PREFACE. 

ALTHOUGH  electro-metallurgy  is  still  in  the  early  stages 
of  development,  no  apology  is  needed  for  the  devotion 
of  a  volume  to  the  application  of  electricity  to  the 
melting  and  treatment  of  steel. 

The  electric  furnace  gives  us  a  new  atmosphere,  in 
which  steel-making  can  be  accomplished.  Purification 
of  miscellaneous  qualities  of  steel  containing  noxious 
substances  can  be  controlled  with  great  exactitude. 
Scientific  synthesis  of  complex  steels  by  the  addition 
of  many  valuable  elements  to  pure  metallic  iron  under 
accurate  chemical  control  has  replaced  the  old  methods 
which  are  sometimes  suggestive  of  the  mysterious  prac- 
tices of  the  alchemists. 

The  melting  of  steel  in  a  truly  reducing  atmosphere 
is  only  possible  with  electric  heat,  and,  consequently, 
new  phenomena  are  observed  and  often  striking  results 
obtained.  Some  of  these  we  understand,  or  suppose  we 
do,  but  others  must  be  the  subject  of  further  research. 

The  electric  furnace  has  given  us  at  reasonable  prices 
new  materials  of  special  value  to  the  steel-maker,  such 
as  low  carbon  ferro-chrome  and  high  grade  ferro-silicon, 
which,  in  turn,  have  been  economically  made  into  such 
products  as  stainless  steel  and  transformer  iron.  The 
former  product  is  being  used  by  engineers  for  remark- 
able new  applications,  and  the  latter  has  increased  the 
efficiency  of  electric  transformers  to  an  extent  which 
represents,  an  annual  saving  of  hundreds  of  thousands 
of  tons  of  coal  per  annum. 

The  electric  furnace  came  as  a  corollary  to  the  con- 
struction of  the  first  dynamos,  and  although  only  twenty 
years  old,  it  is  already  absorbing  millions  of  electrical 

5001-12 


VI  PEEFACE 

horse-power  for  various  purposes,  and  has  produced 
more  than  a  million  tons  of  steel. 

The  widest  application  of  this  process  in  the  imme- 
diate future  will  probably  be  for  the  treatment  of  the 
phosphoric  ores  of  Alsace-Lorraine,  where  the  process 
of  Thomas  and  Gilchrist  will  be  supplemented  by  electric 
refining.  The  stringent  requirements  of  modern  engi- 
neering will  then  be  met  by  electrically  refined  steel  of 
the  highest  quality,  and  the  valuable  phosphates  will  be 
rendered  available  for  the  great  industry  of  agriculture. 
Other  ores  containing  insufficient  phosphorus  for  the 
basic  bessemer  process  may  soon  be  smelted  with  phos- 
phoric limestone,  and  similarly  treated,  so  that  the 
phosphorus  now  wasted  may  be  recovered  and  returned 
to  the  soil. 

To  the  electrician,  the  problem  of  electric  furnace 
design  and  operation  is  of  absorbing  interest.  With 
currents  of  20,000  amperes  or  more,  phenomena  of  re- 
actance, eddy  currents,  and  skin  effect  have  involved 
new  problems  only  recently  recognised,  and  as  yet  not 
fully  appreciated. 

The  author  has  written  a  book  which  should  attract 
both  the  student  and  the  practical  steel-maker,  as  his 
scientific  attainments  and  wide  experience  of  steelworks 
in  several  European  countries  enable  him  to  appreciate 
the  difficulties  of  those  who  have  to  deal  with  the 
electro-metallurgy  of  steel  both  in  its  theoretical  and 
in  its  practical  aspects. 

The  volume  gives  the  early  history  of  a  new  branch 
of  metallurgical  science,  which  is  surrounded  by  many 
fascinating  problems  and  great  promise  of  new  achieve- 
ment, while  a  clear  statement  is  made  of  our  present 
knowledge  of  the  use  of  electric  furnaces  for  the  metal- 
lurgy of  steel. 

DONALD  R  CAMPBELL. 


INTRODUCTION. 

THE  electro-metallurgy  of  steel  as  an  industrial  science 
owes  its  present  status  to  the  vast  development  of  the 
past  few  years.  Prior  to  the  outbreak  of  the  Great 
War,  the  electric  furnace  had  only  a  very  limited  appli- 
cation in  Great  Britain  and  other  countries.  The 
shortage  of  high  grade  raw  materials,  the  enormous 
demand  for  alloy  steels,  and  the  vast  accumulation  of 
heavy  steel  turnings  presented  exceptional  opportunities 
for  the  electric  furnace,  and  it  was  only  then  that  its 
economic  advantages  in  certain  branches  of  steel-making 
began  to  be  generally  recognised.  Since  that  time 
various  modifications  have  been  made  in  the  manner  of 
utilising  electric  energy  in  arc  furnaces,  and  a  far  clearer 
and  wider  understanding  of  the  chemical  reactions 
peculiar  to  the  processes  of  steel- making  in  these 
furnaces  has  also  resulted. 

The  induction  furnace  at  one  time  certainly  received 
greater  prominence  than  the  arc  type,  although  it  is 
doubtful  whether  it  ever  had  a  corresponding  advantage 
in  actual  output  of  steel.  Many  furnace  designers,  in 
their  belief  that  the  principle  of  induction  heating  had 
superior  advantages,  concentrated  their  efforts  on  the 
production  of  a  furnace  which  could  operate  on  any 
standard  electric  supply  system,  and,  at  the  same  time, 
meet  all  the  requirements  of  the  steel-maker.  As  a 
result,  numerous  types  were  introduced,  and  consider- 
able publicity  given  to  their  construction  and  operation. 

vii 


Vlll  INTEODUCTION 

In  this  book,  the  electro-thermic  processes  of  steel- 
making  have  been  studied  more  especially  in  their 
relation  to  arc  furnaces,  from  which  almost  the  entire 
output  of  electric  steel  is  now  being  made.  Little  space 
has,  therefore,  been  devoted  to  the  construction  and 
operation  of  induction  furnaces,  although  sufficient  has 
been  written  to  indicate  their  electrical,  constructional, 
and  metallurgical  features  and  the  principles  involved. 

In  the  earliest  days  of  crucible  steel-making,  the  im- 
portance of  the  character  of  the  fuel  used,  the  method 
of  heat  application  and  temperature  control  were  fully 
recognised  and  studied  by  the  steel-maker;  precisely 
the  same  consideration  is  also  given  to  these  factors 
now  by  those  responsible  for  the  manufacture  of  steel 
in  open-hearth  gas  furnaces.  It  is  obvious  that  a 
full  knowledge  of  both  the  chemical  and  physical  con- 
ditions, which  constitute  a  thermo-chemical  process  for 
the  production  of  a  metal,  should  be  within  the  province 
of  the  metallurgical  engineer,  and  there  is  no  reason 
why  any  such  process  conducted  by  electro-thermic 
means  should  be  an  exception.  Naturally,  it  may  be 
thought  that  the  application  of  electric  energy  to  either 
induction  or  arc  heating  is  hardly  comparable  to  that  of 
solid  or  gaseous  fuel,  and  demands  the  study  of  a  special 
branch  of  electrical  engineering.  This  is  certainly  true 
from  the  point  of  view  of  electric  furnace  design,  but 
does  not  properly  apply  to  the  metallurgist,  who  will 
only  require  a  general  knowledge  of  the  electric  fuel 
provided.  The  metallurgist  best  knows  the  thermal 
and  chemical  conditions  which  conduce  to  maximum 
economy  and  technical  efficiency,  and  should  be  in  a 
position  to  judge  whether  the  electric  energy  is  being 
utilised  in  the  most  suitable  manner. 

The  chemical  and  electrical   conditions  are  always 


INTRODUCTION  ix 

mutually  dependent  upon  one  another,  and,  without  a 
knowledge  of  the  general  principles  underlying  the  use 
of  alternating  currents  for  arc  heating,  it  is  impossible 
to  know  whether  a  slight  adjustment  of  the  electrical 
or  the  chemical  conditions  is  required  to  remedy  an 
unsatisfactory  state  of  either.  The  problem  is,  there- 
fore, more  complex  than  for  those  processes  in  which 
the  thermal  conditions  are  quite  uninfluenced  by  the 
chemical  operations. 

The  general  principles  and  application  of  alternating 
currents  have  been  very  briefly  described  in  Chapters 
If. -IV.,  and  every  endeavour  has  been  made  to  pre- 
sent this  introductory  study  in  a  manner  comprehensible 
to  those  without  any  mathematical  or  other  special 
knowledge  of  the  subject.  This  attempt  probably  falls 
far  short  of  its  object,  and  may  receive  criticism  from 
those  well  acquainted  with  this  special  branch  of 
electrical  engineering.  However,  a  little  knowledge  is 
not  always  dangerous,  particularly  if  it  should  enable 
the  cause  of  certain  phenomena  to  be  correctly  inter- 
preted and  discussed  with  those  who  are  responsible  for 
effecting  the  remedy.  Factors  bearing  upon  the  cost 
and  economic  use  of  electric  energy  have  been  dealt 
with  in  Chapter  VI.,  and  it  is  hoped  that  the  issues 
raised  will  indicate  how  the  maximum  economy  of 
power  cost  may  be  attained. 

It  has  been  impossible  to  give  any  definite  figures 
bearing  upon  the  cost  of  steel  production  as  governed 
by  factors  other  than  the  actual  cost  of  power,  raw 
materials,  and  labour.  It  has  been  emphasised  through- 
out this  book  that  such  all- important  factors  as  power- 
consumption,  life  of  linings,  and  electrode  consumption 
are  not  only  all  mutually  interdependent  for  any  given 
type  of  furnace,  but  are  absolutely  controlled  by  the 


X  INTRODUCTION 

furnace  load  factor  and  the  precise  nature  of  the  process 
used.  For  these  reasons,  even  comparative  costs  are 
dangerous  and  misleading  without  stating  the  exact 
conditions  of  operation  in  each  case,  and  it  is,  therefore, 
far  better  to  ascertain  such  figures  under  the  exact 
working  conditions  provided  for  from  any  of  the 
numerous  sources  available. 

The  author  has  to  express  his  great  indebtedness 
to  Mr.  R  P.  Abel  for  the  unstinted  assistance  he  has 
given  in  connection  with  the  electrical  chapters,  while 
his  thanks  are  also  due  to  Messrs.  Thwaites  Bros,  for 
affording  every  facility  to  investigate  their  plant,  and 
for  providing  several  of  the  illustrations  ;  to  Messrs. 
B.  Drinkwater  and  J.  Holden  for  their  assistance 
in  compiling  the  Appendix  ;  to  those  constructors  of 
electric  furnaces  who  have  kindly  placed  at  his  disposal 
information  and  drawings  relating  to  their  operation 
and  design  ;  and  to  numerous  friends  who  have  given 
advice  and  information. 

The  majority  of  the  illustrations  are  original, 
and  permission  to  reproduce  many  of  the  remainder 
was  kindly  granted  by  The  Iron  and  Steel  Institute, 
The  Faraday  Society,  The  American  Electro-chemical 
Society,  and  the  Canadian  Government  Commission. 

Lastly,  the  author  desires  to  express  his  grateful 
acknowledgment  to  Mr.  I).  F.  Campbell  for  contributing 
the  Preface. 

C.    C.    GOW. 


CONTENTS. 

CHAPTER  I. 

PAGE 

HISTORICAL  DEVELOPMENT  OF  ELECTRIC  STEEL  FURNACES.        .        .        .1 

CHAPTER  II. 
DEFINITIONS  OF  ALTERNATING  CURRENT  CHARACTERISTICS  .        .        ^        .       42 

CHAPTER  III. 
APPLICATION  OF  SINGLE  AND  POLYPHASE  CURRENTS     .        .        .       *.        .      52 

CHAPTER  IV. 
GENERATION  AND  CONTROL  OF  SINGLE  AND  POLYPHASE  CURRENTS      .        .      74 

CHAPTER  V. 
AUTOMATIC  REGULATORS  AND  ACCESSORY  INSTRUMENTS         .        .        .        .86 

CHAPTER  VI. 
POWER  CONSUMPTION  AND  CONTRIBUTORY  FACTORS 102 

CHAPTER  VII. 
ELECTRO-METALLURGICAL    METHODS    OF    MELTING    AND    REFINING    COLD 

CHARGES    .        .        .        .        .        .        .        . . 113 

CHAPTER  VIII. 
LIQUID  STEEL  REFINING •.".-.        •     157 

CHAPTER  IX. 
INGOT  CASTING .        .        .        .     165 

CHAPTER  X. 
APPLICATION  OF  THE  ELECTRIC  FURNACE  TO  FOUNDRY  PRACTICE        .        .     197 

CHAPTER   XI. 
CHARACTERISTIC  PRINCIPLES  AND  FEATURES  OF  ELECTRIC  FURNACE  DESIGN    214 

CHAPTER   XII. 
MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES    ....  .     234 


Xll  CONTENTS 

CHAPTER   XIII. 

PAGE 

REFRACTORY  MATERIALS    AND  THEIR  APPLICATION  TO  ELECTRIC  FURNACE 

CONSTRUCTION 281 

CHAPTER  XIV. 
FURNACE  LINING  AND  LINING  REPAIRS 292 

CHAPTER   XV. 
PROPERTIES  AND  MANUFACTURE  OF  CARBON  ELECTRODES     ....     306 

APPENDIX  I. 
RAPID  METHODS  OF  ANALYSIS  FOR  BATH  SAMPLES       .....     330 

INDEX  343 


LIST   OF  ILLUSTRATIONS. 


CHAPTEK  I. 

FIG.  NO.  PAGE 

1.  Siemens' Direct  Arc  Furnace         .        ;        .....        .  ,     '  2 

2.  Siemens'  Indirect  Arc  Furnace      .        .        .        .        .        .        .  .2 

3.  Ferranti  Induction  Furnace  .         .         .        .        .        .        .        .  ."8 

4.  Ferranti  Induction  Furnace  .         .         .        .        .        .        .        .  .4 

5.  Colby  Induction  Furnace       .         .         .                 .        .        .        .  .        5 

6.  Kjellin  Fixed  Induction  Furnace  (Horizontal  Section)  .         .        .  .        6 

7.  Kjellin  Fixed  Induction  Furnace  (Vertical  Section)       .        .        .  .7 

8.  KjeUin  Tilting  Furnace  (Vertical  Section)     .        ..'...  .        8 

9.  Rflchling-Rodenhauser  Single-phase  Furnace  (Horizontal  Section)  .        9 

10.  Rochling-Rodenhauser  Single-phase  Furnace  (Vertical  Section)  .  .       10 

11.  Rochling-Rodenhauser  Three-phase  Furnace  (Horizontal  Section)  .       11 

12.  Rochling-Rodenhauser  Three-phase  Furnace  (Vertical  Section)   .  .       11 

13.  Frick  Induction  Furnace  (Vertical  Section)   .         ...        .  .12 

14.  Frick  Induction  Furnace  (Front  Elevation)  .         .        .        .        .  .14 

15.  Frick  Induction  Furnace  (Plan)     ........       15 

16.  Hiorth  Induction  Furnace .  .       16 

17.  Stassano  Smelting  Furnace '.  .      17 

18.  Stassano  Smelting  Furnace  (later  type)         ...        .        .        .  .      18 

19.  Stassano  Smelting  Furnace  (later  type)         .        .        .        .  .19 

20.  Heroult  Fixed  Furnace  (earlier  form)    .  *      .        .        .        .        .  .      23 

21.  Heroult  Tilting  Furnace         .        , 24 

22.  Heroult  Tilting  Furnace  at  La  Praz  (France)       .        .      Facing  page      25 

23.  Keller  Furnace  at  Unieux  (France)        .        .        .        .        .        .  .27 

24.  Keller  Furnace  (earlier  type) .         .        .        .    '     ,        .         ...  .29 

25.  Keller's  Conducting  Hearth  .        .      .....        .        .  .  .       30 

26.  Keller's  Radiating  Conductors .32 

27.  Girod  Bottom  Conductors       .        .        ."....        .        .  .34 

28.  Girod  Annular  Conductor       .        .        .        .        .-       .        .        .  .       34 

29.  Electro-metal  Tilting  Furnace       .        ...        .        .        '.  .38 

30.  Snyder  Furnace      .        .        .        .        .        .        .        .        .        .  .      40 

CHAPTER  II. 

31.  Wave  Form  and  Clock  Diagram    ....        .        .        .        .  .      42 

32.  Clock  Diagram  with  Two  Vectors  .        .        , 43 

33.  Wave  Forms  for  Two  Vectors         .        *     -  .        .        .  -     .        ,  .       43 

34.  Conductors  and  Magnetic  Field     .         .         ...»        .        .  .46 

35.  Closed  Ring  and  Two  Circuits        .        ,      ,.        .        .......       46 

36.  (Two-phase  Voltage)  Wave  Form  Diagram    .        . -.  ;•  ,        i  •    ..V  •       ^ 

xiii 


XIV  LIST   OF   ILLUSTRATIONS 

CHAPTER  III. 

FIG.  NO.  PAGE 

37.  (Three-phase  Voltage)  Wave  Form  Diagram 51 

38.  Series  Arc  Single-phase  Connections 52 

39.  Single  Direct  Arc  Connections 52 

40.  Single  Indirect  Arc  Connections 53 

41.  Induction  Furnace  Connections 53 

42.  Three  Wire  Single-phase  Connections .         .53 

43.  Two- phase  Circuits 54 

44.  Two-phase  Wave  Curves  and  Resultant 54 

45.  Heroult  Two-phase  Connections 55 

46.  Electro-Metals  Two-phase  Connection 56 

47.  Electro-Metals  Two-phase  Connection 56 

48.  Girod  Two-phase  Connections 56 

49.  Stobie  Two-phase  Connections 57 

50.  Rennerfelt  Two-phase  Connections 58 

51.  Dixon  Two-phase  Connections        .         .         .         .         <         .         .         .58 

52.  Three-phase  Wave  Curves .59 

53.  Three-phase  Circuits 60 

54.  Star  Connection  Diagram       .         . 60 

55.  Three-phase  Nomenclature  Diagram .62 

56.  Three-phase  Clock  Diagram 63 

57.  Three-phase  Circuits 63 

58.  Mesh-connected  grouping       .........  63 

59.  60°  Three-phase  Wave  Curves 65 

60.  Heroult  Three-phase  Connections 67 

61.  Three-phase  Inverted  Star      .         . 67 

62.  Four-phase  Wave  Curves 69 

63.  Dixon  Four-phase  Star  Connection 70 

64.  Dixon  Four-phase  Mesh  Connection 71 

65.  Electro-Metal  Four-phase      .         .                  72 

66.  Resultant  of  Unequal  Waves  (120°  apart)       .         .         .         .         .         .72 

CHAPTER  IV. 

67.  Miles-Walker  Generator,  Characteristic  Curve 76 

68.  Scott  Two-phase  to  Three-phase  Transformer  Connections   ...  80 

69.  Scott  Three-phase  to  Two-phase  Transformer  Connections   ...  80 

70.  Change-voltage  Switch  (Mesh  Connections) 81 

71.  Change-voltage  Switch  (Star  Connections) 81 

72.  High  Tension  Reactance  with  Tappings 82 

73.  Circuit  Reactance  and  Resistance  Volts 84 

74.  Circuit  Reactance  and  Resistance  Volts 84 

CHAPTER   V. 

75.  Thury  Regulator Facing  page  90 

76.  Thury  Regulator,  Diagram  of  Connections 94 

77.  Curve  of  Load  Factor  and  Cost  per  Unit 106 

78.  Curve  of  Operating  Load  Factor  and  Useful  Energy      ....  109 

79.  Curve  of  Monthly  Load  Factor  and  Power  Consumption  and  Curve  of 

Monthly  Loa'd  Factor  and  Weekly  Output 110 


LIST   OF   ILLUSTRATIONS  XV 

CHAPTER    VII. 

PIG.  NO.  PAGE 

80.  Pouring  Spout  to  Hold  Back  Slag 148 

81.  Furnace  Tools 149 

CHAPTER  IX. 

82.  Illustration  of  Solidification 169 

83.  Fracture  Showing  Chill  and  Equixed  Crystals      .  .      .       Facing  page    170 

84.  Solidification  of  Steel  in  Parallel  Mould        .         .  '      .        .         .         .172 

85.  Tilting  Ladle          .         .         .      .  .         .         .        .       ..     :.  .        .        .     182 

86.  Bottom  End  of  Built  Up  Stopper  Rod  .         ..       .       , .        .        .        .     183 

87.  Ladle  Heating  Furnace          .         .         .         ......        .185 

88.  Ladle  Heating  Blast  Pipe-     .........     186 

89.  Ingot  Section  Cast  Small  End  up  ...        .        .        .        .     189 

90.  Ingot  Section  Cast  Big  End  up       .         .         .       -.         .         .        .        .189 

91.  Closed  Top  Mould  and  Bottom  Plate     .        ,'.       .  '   ~:.    '.   .        ,        .     191 

92.  Four- Way  and  Six- Way  Runner  Bricks          .     •   .         .   -     .        .        .     192 

93.  Trumpet  Pipes        .         .         .        .        .        .         .         .  '      ...     192 

94.  Mould  for  Bottom-running  (Big  end  up)         .  .        .         .         .     192 

95.  Dozzles  .        .       . .         .         .         .         .         ....        .         .195 

96.  Cheek  Bricks.         .         ....        'i        .        ...        .195 

97.  Tun  Dish '/     .       :.        .        .        .         .         .     196 

CHAPTER  X. 

TABLE  1. — ADVANTAGES  AND  DISADVANTAGES  OF  CRUCIBLE,  CONVERTER, 
AND  ELECTRIC  FURNACE. 

98.  Furnace  Installation  in  Foundry  .        .        .         .        .       Facing  page    204 
69.     Microphotographs  Showing  Effect  of  Annealing  .        .  „        „        208 

100.  Microphotographs  Showing  Effect  of  Annealing    .         .  „        ,,        208 

101.  Slag  Inclusions       .        . ..-••'.        .        .         .         .         .  ,.         ,.         212 

CHAPTER  XII. 

102.  Section  of  Stassano  Furnace  Melting  Chambers   .        .        .        .        .235 

103.  Vertical  Section  of  Stassano  Furnace    ...--.        .        .        .         .235 

104.  Rennerfelt  Furnace  Wiring  Diagram     .        .        .        .        .        .        .     238 

105.  Rennerfelt  Furnace  Section  of  Trunnion  Mounted  Body     Facing  page    240 

106.  Rennerfelt  Furnace  (rectangular) .         .  •.        .         .         .     241 

107.  Rennerfelt  Furnace.     (Section  Through  a  Basic  Lining)       .         .         .     242 

108.  Heroult  Three-phase  Furnace  (6  Tons  capacity) 248 

109.  Heroult  One  and  a  Half  Ton  Furnace  .        ,        .  .         .        .     251 

110.  Girod  Single-Phase  Diagrams        .        .         .        .        .        .         .         .     252 

111.  Girod  Single-Phase  Furnace  ...        .        .        .        ..     %.         .253 

112.  Electro-Metals  Wiring  Diagram Facing  page     256 

113.  Electro-Metals  Seven  and  a  Half  Ton  Furnace     .  „        257 

114.  Electro-Metals  Four-phase  Furnace      ,        .        .        .         ...     258 

115.  Stobie  Five  Ton  Two-phase  Furnace      .......     261 

116.  Snyder  Power  and  Current  Curves         ...        ....     264 

117.  Snyder  Furnace  Load  Chart .         .        .        .        .        .        .        .         .     266 

118.  Snyder  Five  Ton  Furnace      .        .        .  .        *    .    .        .        .     268 

119.  Snyder  Furnace  Section      .    .        .        «        .         ...        .        .269 


XVI  LIST   OF   ILLUSTRATIONS 

FIG.  NO.  PAGE 

120.  Greaves-Etchells  Twelve  Ton  Furnace  Section 272 

121.  Greaves-Etchells  Twelve  Ton  Furnace  Section  (Back  Elevation)  .         .  274 

122.  Booth-Hall  Furnace  Connections 275 

123.  Booth-Hall  Furnace  (Transverse  Sections) 278 

CHAPTER   XIV. 

124.  Key  Sketches  for  Lining  Furnace 294 

125.  Lining  of  Seven  Ton  Furnace 295 

12G.     Lining  of  Conductive  Hearth  Furnace 298 

CHAPTER   XV. 

127.  Diagram  of  Apparatus  for  Electrode  Conductivity  Test        .         .         .  316 

128.  Double  Electrode  Screw  Plug 319 

129.  Stobie  Economiser 327 

130.  Electro-Metals  Economisers  .         . 328 

131.  Ball  and  Socket  Sleeve  Economiser 328 

APPENDIX   I. 

132      Carbon  Combustion  Apparatus Facing  page  340 


CHAPTER  I. 

HISTOKICAL  DEVELOPMENT  OF  ELECTRIC  FUENACES. 

THE  Electro-metallurgy  of  steel  is  now  an  applied  science  of 
considerable  industrial  importance,  and  has  only  reached  its 
present  stage  of  development  during  comparatively  recent  years. 
The  application  of  electric  energy  for  melting  steel  had  been 
demonstrated  by  Sir  William  Siemens  almost  twenty  years 
before  its  commercial  possibilities  were  recognised  by  later  in- 
vestigators, to  whose  work  the  present  status  of  the  electric  steel 
industry  is  primarily  due. 

The  generation  of  electric  currents  by  means  of  the  dynamo 
or  similar  device  was  only  discovered  in  1867,  so  that  the  slow 
advancement  made  subsequent  to  the  work  of  Siemens  was  no 
doubt  mainly  due  to  the  lack  of  thoroughly  reliable  electrical 
equipment,  without  which  any  electro-chemical  process,  though 
conducted  with  the  best  technical  and  organising  skill,  is 
doomed  to  commercial  failure. 

The  use  of  electrical  heating  for  melting  steel  on  a  com- 
mercial scale  was  first  demonstrated  by  the  induction  furnace, 
but  no  real  impetus  was  given  to  the  industry  until  the 
introduction  of  the  arc  furnace  a  few  years  later.  For  several 
succeeding  years  the  electro-thermic  processes  were  limited  to 
localities  peculiarly  favourable  to  economy  of  power  production 
and  to  a  few  large  undertakings,  which,  at  the  instigation  of  the 
inventors,  constructed  larger  units  for  the  further  study  of  the 
products  and  cost  of  production. 

Before  investigating  further  the  development  of  the  electric 
steel  furnace  at  this  period,  the  work  of  Sir  William  Siemens 
in  1882  must  be  more  closely  considered.  The  melting  of  steel 
was  accomplished  in  a  small  crucible  furnace  in  which  the 
principles  of  both  direct  and  indirect  arc  heating  were  utilised 

1 


^LECTRO-METALLURGY   OF    STEEL 


i  gtstvest&bUslied;f  .Figs.  1  and  2  illustrate  the  two  arrange- 
ments used.     In  the  case  of  the  former  (Fig.  1),  the  furnace 

charge  serves  as  one  electrode,  and  is 
connected  to  the  source  of  electrical 
supply  through  a  carbon  or  metallic 
pole  penetrating  the  furnace  bottom  or 
hearth.  The  electric  current  enters 
the  furnace  through  an  upper  hanging 
electrode,  and  strikes  an  arc  at  the 
point  of  the  conducting  charge  where 
contact  is  made.  The  principle  in- 
volved is  similar  to  that  embodied  in 
several  modern  types  of  furnaces.  The 
furnace  represented  in  Fig.  2  is  de- 
pendent on  indirect  arc  heating;  and 
is  furnished  with  horizontal  electrodes 
which  pass  through  the  walls.  The 
arc  is  struck  between  the  ends  of  the  electrodes,  and  the 
furnace  charge  receives  heat  by  radiation  from  the  arc  and 
reflection  from  the  roof  and  .walls.  Here,  again,  the  principle 


FIG.  1. — Siemens'  direct  arc 
furnace. 


FIG.  2. — Siemens'  indirect  arc  furnace. 

is  identical  with  that  employed  in  another  class  of  modern 
electric  furnaces  of  which  the  Stassano  furnace  was  the  fore- 
runner. 

To  Sir  William  Siemens,   therefore,  belongs  the  credit   of 
having  first  demonstrated  two  systems  of  electric  arc  heating  as 


HISTORICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES  3 

applied  to  steel  melting,  and  in  this  respect  he  may  be  con- 
sidered the  pioneer  of  the  industry.  His  researches  were 
certainly  confined  to  the  laboratory  scale,  but  it  is  more  than 
probable  that,  had  the  requisite  electrical  plant  been  available 
at  that  time,  his  efforts  would  have  been  further  extended  to 
the  commercial  application  of  his  discovery. 

A  period  of  inactivity  in  the  development  of  arc  heating 
followed  until  1898,  but  during  that  time  other  discoveries 
were  made  which  demonstrated  the  possibility  of  melting 
metals  by  the  heating  effect  of  low  voltage  currents  induced 


FIG.  3. — Ferranti  induction  furnace. 

in  the  metal  itself.     The   electrical  principles  involved  in  the 
operation  of  induction  furnaces  are  dealt  with  in  Chapter  II. 

Ferranti 1  in  1887  constructed  a  furnace  on  this  principle  in 
which  he  was  able  to  melt  metals,  although  no  special  claim 
was  made  for  melting  steel.  The  principle  of  induction  heating 
was  later  adopted  in  various  furnaces  which  at  one  time  promised 
to  bear  an  important  part  in  electric  steel  melting,  but  which 
have  now  been  almost  entirely  abandoned  in  favour  of  arc 
furnaces. 

The  melting  chamber  of  Ferranti's  furnace  (Figs.  3  and  4) 
consists  of  an  annular  channel  A,  provided  with  a  suitable  cover- 

1  British  Patent  Specification,  No.  700,  1887. 


4  THE   ELECTKOMETALLURGY   OF    STEEL 

ing,  surrounding  the  centre  limb  of  a  double  closed  magnetic 
circuit  C.  Primary  windings  B  are  also  wound  round  the  same 
limb  immediately  below  the  melting  chamber.  The  apparatus 
consists  essentially  of  a  simple  transformer,  whose  secondary 
winding  of  one  short-circuited  coil  is  composed  of  a  ring  of  the 
metal  to  be  melted.  The  voltage  of  the  induced  current  is 
exceedingly  low,  owing  to  the  low  resistance  of  the  secondary  cir- 
cuit, and  the  intensity  of  the  current  correspondingly  high.  The 
transformer  core,  consisting  of  laminated  sheets  of  iron,  is 
mounted  in  a  framework  which  tilts  on  trunnions,  so  that  the 


FIG.  4. — Ferranti  induction  furnace. 

molten  metal  can  be  easily  poured.  Three  years  later  Ferranti 
was  followed  by  Edward  Colby,1  who  took  out  several  patents  in 
America  for  melting  metals  in  induction  furnaces.  In  his  original 
type  the  primary  winding  surrounded  the  secondary  or  annular 
melting  chamber,  but  this  arrangement  was  later  given  up  and 
their  respective  positions  reversed.  Means  were  also  provided  for 
tilting  the  furnace  and  pouring  the  metal  direct.  At  least  eight 
years  elapsed  before  the  first  steel  was  made  (1898-1899)  in  an 
induction  furnace  by  Colby  and  Dr.  Waldo  in  America,  and 

111  El.  Chem.    Industry,"   Vol.   V,   p.    55;    U.S.A.   Patent   Specification,    No. 
428,378,  428,379,  and  428,552. 


HISTOKICAL   DEVELOPMENT   OF   ELECTEIC    FURNACES 


although  several  of  these  furnaces  were  later  installed  for  the  pro- 
duction of  electric  steel,  the  introduction  of  modified  types  soon 
gained  prominence.  One  of  the  distinctive  features  of  the  later 
Colby  furnace l  (Fig.  5)  is  the  double  magnetic  circuit  com- 
prised of  three  vertical  legs  connected  together  by  horizontal 
members.  Both  the  primary  and  secondary  circuits  surrounded 
the  centre  leg  and  were  themselves  enclosed  between  the  two  outer 
legs.  The  primary  windings,  it  will  be  seen,  are  as  close  as  possible 
to  the  centre  core,  and,  by  means  of  suitable  water  cooling 
arrangements,  the  secondary  circuit  or  melting  chamber  could 
be  constructed  in  close  proximity  to  it,  thus  reducing  magnetic 
leakage  to  a  minimum  and  increasing  the  power  factor.  This 
was  undoubtedly  an  excellent  feature,  but  the  difficulty  of 


FIG.  5. — Colby  induction  furnace. 

embodying  this  construction  in  larger  furnaces  (i.e.  above  3 
cwts.)  was  considerable  on  account  of  the  high-voltage  current 
necessary  for  the  supply  of  increased  power  to  the  primary 
windings,  which  would  then  require  elaborate  insulation. 
The  power  factor  of  the  furnace  is  given  by  Colby  as  about  '9, 
which  compares  favourably  with  modern  arc  furnaces. 

Colby's  induction  furnace,  as  actually  applied  to  steel-making, 
was  patented  in  America  in  1900  2  or  at  least  one  year  after  his 
first  production  of  steel.  The  same  year  was  marked  by  further 
improvements  in  induction  furnaces,  and,  what  is  far  more  im- 
portant, by  the  re-introduction  of  arc  heating,  both  direct  and 
indirect,  for  steel  melting.  Thus  the  year  1900  may  be  regarded 

1  U.S.A.  Patent  Specification,  No.  859,641  (1907). 

2  "El.  Chem.  Industry,"  Vol.  Ill,  1905,  pp.  80,  134,  299,  and  341,  also  Vol.  V, 
1907,  p.  232. 


6  THE   ELECTRO-METALLURGY   OF   STEEL 

as  the  most  important  in  the  history  of  the  electric  steel  industry. 
To  avoid  confusion  the  further  development  of  the  induction 
furnace  will  be  first  considered,  returning  later  to  the  history  of 
arc  furnaces,  which  in  their  commercial  form  date  from  this 
year. 

The  Kjellin  induction  furnace,  first  patented  in  1900  and 
operating  in  1902,  embodied  the  same  electrical  principles  as  used 


FIG.  6.— Section  AB. 

by  Ferranti  and  Colby,  the  salient  difference  lying  in  the  use  of 
high-voltage  current  in  the  primary  windings  for  the  purpose  of 
increasing  the  furnace  power  capacity.  One  of  the  magnetic 
circuits  was  also  eliminated,  leaving  of  the  three  vertical  legs 
the  centre  core  and  one  outer  leg,  which  were  set  in  the  plane 
of  tilting.  Figures  6  and  7  show  a  diagrammatic  plan  and 
section  of  the  fixed  furnace  in  operation  at  Gysinge  in  Sweden, 


HISTORICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES  7 

which  was  reported  upon  by  the  Canadian  Government  Com- 
mission in  February,  1903.  The  melting  chamber  B,  when 
charged  with  metal,  constitutes  a  short-circuited  secondary  wind- 
ing of  one  turn,  and  surrounds  the  primary  windings  A  and  one 
limb  of  the  iron  core  C.  The  primary  windings  being  carefully 
insulated  were  supplied  with  a  single-phase  current  of  90  amperes 
at  3000  volts  and  15  cycles,  representing  about  168  kw.  at 
a  power  factor  of  "62.  They  were  further  protected  from  the 


4_  _i 


FIG.  7. 

heat  radiated  from  the  melting  chamber  by  means  of  water  cir- 
culation supplementing  natural  air  cooling.  The  melting 
chamber  was  closed  in  by  a  number  of  small  cover  bricks  KK, 
which  could  be  easily  removed  for  purposes  of  charging,  repair- 
ing and  inspection,  and  was  provided  with  a  lining  composed 
of  silica  or  magnesite  bricks  filling  the  portion  marked  D  set 
on  a  foundation  of  firebricks  D1.  Sufficient  metal  was  always 
left  in  the  furnace  after  tapping  to  provide  an  unbroken  secondary 
circuit  for  the  induced  current  when  starting  a  subsequent  heat. 


8 


THE   ELECTKO-METALLUKGY   OE   STEEL 


The  radiation  loss  of  the  furnace  mentioned  was  equivalent  to 
80  kw.,  leaving  only  88  kw.  for  melting,  and  with  such  a  ratio 
the  power  consumption  could  not  possibly  have  been  anything 
but  high.  The  furnace  was  chiefly  used  commercially  for  melt- 
ing Swedish  white  iron  and  steel  scrap  requiring  no  refining 
treatment.  In  this  respect  any  advantage  of  the  induction 
furnace  over  crucible  melting  lay  solely  in  economy  of  heat 
utilisation. 

The  later  type  of  Kjellin  furnace  was  adapted  for  tilting  and 
is  illustrated  in  Fig.  8.     The  low  frequency  required  necessitated 


FIG.  8. — Kjellin  tilting  furnace. 

the  use  of  special  generating  plant,  and  consequently  increased 
the  cost  of  production  by  the  heavier  capital  outlay  involved. 
The  power  factor  of  the  furnace  was  unavoidably  low,  being 
about  '6  to  '65,  and  fell  still  further  on  increasing  the  furnace 
capacity.  For  this  reason  a  lower  frequency  than  15  was 
necessary  for  the  larger  units.  In  1909  there  were  in  operation 
only  ten  Kjellin  and  one  Colby  furnace,  so  that  ten  years  after 
the  introduction  of  the  induction  furnace  no  considerable  pro- 
gress had  been  made.  The  first  Kjellin  furnace  to  operate  in 
England  was  erected  by  Vickers  Sons  &  Maxim  and  began 
operation  in  1908,  but  this  did  not  lead  to  further  development. 


HISTOEICAL  DEVELOPMENT   OF   ELECTKIC  FURNACES 


9 


Although  at  that  time  considerable .  advances  had  been  made  in 
other  countries  with  induction  and  more  especially  arc  furnaces, 
this  Kjellin  furnace  was  the  first  electric  furnace  plant  to 
be  installed  in  the  United  Kingdom.  A  fixed  furnace  l  of  1  ton 
capacity,  operating  at  Gysinge,  produced  950  tons  of  tool  steel 
and  special  steel  ingots  during  the  year  1906.  The  bulk  of 
the  output  was  produced  from  charges  composed  of  about  80 
per  cent.  Swedish  white  iron  and  20  per  cent,  of  steel  scrap; 
briquettes  of  ore  were  added  to  regulate  the  percentage  of 


carbon  in  the  steel  cast.  The  energy  consumption  2  was  nor- 
mally 1128  kw.  hours  per  ton,  but  when  white  iron  and  steel 
scrap  were  melted  without  briquettes  the  power  consumption 
fell  to  886  units  per  ton. 

A  distinct  departure  from  the  design  of  Kjellin  was  made  in 
the  Kochling-Rodenhauser  furnace,  which  was  patented  in  May, 
1906,  and  designed  to  operate  with  either  single  or  three-phase 
current.  In  each  case  the  iron  cores  and  primary  windings  were 
enclosed  by  a  secondary  circuit.  Auxiliary  secondary  windings, 

1  "Iron  and  Steel  Institute  Jour  .al,"  Vol.  Ill,  1906,  p.  397. 

2  Ibid.,  Vol.  I,  1909,  p.  298. 


10 


THE   ELECTRO-METALLURGY   OF   STEEL 


consisting  of  a  few  turns  of  heavy  copper  bar,  were  connected  to 
terminal  plates  embedded  in  the  furnace  lining,  and  provided  a 
further  source  of  heat  to  the  furnace  charge,  besides  improving 
the  electrical  efficiency.  Figs.  9  and  10  l  show  a  sectional  plan 
and  elevation  of  the  single-phase  furnace,  and  Figs.  11  and  12 
corresponding  views  of  the  three-phase  design. 

Two  furnaces,  one  of  which  was  an  8- ton  single-phase  furnace 
and  the  other  a  2-ton  three-phase  furnace,  were  being  operated 
at  the  Rochling  Iron  and  Steelworks  at  Volklingen  in  1909.  The 
8-ton  furnace  of  600  kw.  capacity  was  supplied  with  power  from 


a  4000  to  5000  volt  5  cycle  per  second  generator,  while  the 
smaller  three-phase  furnace  of  250  kw.  capacity  operated  on  a 
supply  of  400  volts  at  50  cycles.  The  higher  periodicity  per- 
missible with  the  three-phase  furnace  results  from  (a)  its  smaller 
capacity,  (b)  the  higher  power  factor  obtainable  by  the  closer  and 
better  arrangement  of  the  transformer  cores  relative  to  the 
primary  and  secondary  windings,  (c)  the  auxiliary  secondary 
windings  which  reduce  magnetic  leakage. 

In  both  furnaces  the  annular  melting  chambers  meet  in  a 
common  hearth  of  wider  proportions,  situated  between  the 
vertical  legs  of  the  iron  core,  and  it  is  here  that  the  heat  is 

1 "  Iron  and  Steel  Institute  Journal,"  Vol.  I,  1909,  p.  270. 


HISTORICAL   DEVELOPMENT   OP  ELECTKIC   FURNACES         11 

intended  to  be  absorbed  by  the  current  generated  in  the  auxiliary 


FIG.  11. 


FIG.  12. 


secondary  windings.     The  increased  dimensions  of  this  hearth 
enabled  certain  metallurgical  operations  to  be  performed  which 


12 


THE   ELECTEO-METALLUEGY    OF    STEEL 


had  hitherto  been  impossible  in  the  older  forms  of  induction 
furnaces.  The  central  chamber  is  arched  over  (Figs.  10  and 
12),  and  provided  with  a  charging  door  and  spout.  Fluxing 
materials  can  thus  be  easily  charged  and  the  bulk  of  slag 
removed  if  desired.  The  greater  part  of  the  heat  is  generated 
in  the  narrowed  section  of  the  secondary  channels,  and  trans- 
mitted to  the  metal  in  the  centre  portion  by  circulation.  The 


FJG.  13. 

auxiliary  heating  already  mentioned  is  of  doubtful  advantage, 
and  it  is  not  clear  how  the  bulk  of  this  heat  can  be  generated 
anywhere  but  in  the  refractory  lining  separating  the  terminal 
plates  from  the  metal.  The  circulation  of  metal  along  the 
narrow  portion  of  the  secondary  circuit  is  promoted  by  electro- 
magnetic effects  in  a  fixed  direction,  and  maintains  a  sufficiently 
high  temperature  in  the  centre  portion  of  the  bath  to  promote 


HISTORICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES          13 

slag  reaction  and  for  casting.  According  to  Kodenhauser  the 
single-phase  furnace,  when  under  three  tons  capacity,  is  superior 
as  regards  efficiency  to  the  polyphase,  while  for  larger  sizes  the 
polyphase  type  is  preferred.  This  is  mainly  due  to  the  power 
factor  of  the  single-phase  furnace  becoming  increasingly  lower 
as  the  distance  of  the  secondary  melting  chamber  from  the 
iron  core  becomes  greater. 

From  a  metallurgical  standpoint,  the  "  Combination " 
furnace,  as  it  was  called  by  Rodenhauser,  was  a  material 
advance  on  the  simple  induction  furnace ;  not  only  could  pure 
materials  be  melted  and  poured,  but  the  construction  of  the 
hearth,  as  previously  explained,  permitted  the  removal  of  phos- 
phorus and  sulphur  from  less  pure  classes  of  scrap.  In  the 
year  1913 l  there  were  thirteen  Rochling-Rodenhauser  furnaces 
actually  in  operation,  ten  of  which  were  employed  for  refining 
molten  steel  or  pig-iron,  and  the  remaining  three  for  melting 
and  refining  cold  charges. 

Two  years  after  the  introduction  of  the  Rochling-Roden- 
hauser furnace  another  induction  type  furnace,  designed  by 
Frick,1  was  operating  at  Krupp's  Essen  works  with  a  power 
input  of  750  kw.  The  single-phase  furnace,  as  illustrated  in 
Figs.  13,  14,  and  15,  has  certain  features  in  which  it  resembles 
both  the  Colby  and  Kjellin  furnaces.  A  three-legged,  laminated 
iron  core  is  adopted,  and  the  primary  windings  are  designed 
for  high-voltage  currents.  The  peculiar  construction  of  the 
primary  windings  was  intended  to  improve  the  power  factor  by 
reducing  magnetic  leakage,  and  to  effect  this  object  the  coils 
were  split  up  into  three  portions,  the  top  and  bottom  coils  being 
flattened  out  to  enclose  the  secondary  circuit.  This  was  com- 
monly known  as  the  "Umbrella"  design.  Side  inspection 
doors  were  also  provided  in  place  of  the  removable  cover  bricks 
of  the  earliest  Kjellin  pattern.  Frick,  in  his  paper  read 
before  the  Iron  and  Steel  Institute  in  1913,  describes  at  con- 
siderable length  the  electrical  and  metallurgical  operation  of 
his  furnace,  but  does  not  indicate  any  considerable  improve- 
ment in  power  factor  resulting  from  the  special  arrangement 
of  the  primary  windings.  The  annular  melting  chamber, 

1 ':  Iron  and  Steel  Institute  Journal,"  Vol.  II,  1913,  pp.  297  et  seq. 


14 


THE    ELECTRO-METALLUEGY   OF    STEEL 


necessarily  steep-sided,  was  composed  of  a  very  pure  magne- 
site  treated  in  such  a  way  as  to  resist  contraction  and  ex- 
pansion at  high  temperatures.  This  lining  enabled  charges 


FIG.  14. — Frick  induction  furnace. 

to  be  melted  down  under  basic  slags  for  purposes  of  phos- 
phorus removal.  Further,  a  rotatable  cover  was  provided 
for  the  melting  chamber  to  facilitate  uniform  charging  and 
repairing. 


HISTORICAL   DEVELOPMENT    OF   ELECTRIC   FURNACES         15 

There  now  only  remains  to  be  mentioned  the  Hiorth1 
furnace  which  was  patented  in  Norway  in  May,  1909,  and  put 
into  operation  early  in  1910.  J.  W.  Eichards,  in  a  paper  read 
before  the  American  Electro-chemical  Society  in  1910,  describes 
a  5-ton  capacity'  unit  supplied  with  400-500  kw.  at  250  volts, 
12-£  cycles.  The  furnace  (Fig.  16)  resembles  the  Eodenhauser 
arrangement  of  a  double  secondary  circuit  joining  to  form  a 


o  . 

i 

FIG.  15. — Frick  induction  furnace. 

central  hearth,  and  embodies  also  the  "  Umbrella "  type  of 
primary  winding  used  by  Frick.  In  this  case,  however,  the 
primary  voltage  is  low,  and  to  enable  the  heavier  current  to  be 
carried  at  greater  current  density  the  primary  coils  are  con- 
structed of  copper  tubing  for  water  circulation.  During  the 
heat  made  on  the  occasion  of  J.  W.  Kichard's  visit  to  the 
Jossingford  works,  the  power  factor  varied  from  '57  to  *8.  A 

1 "  Am.  Electro-Chem.  Soc.,"  Vol.  XVIII,  1910,  p.  191. 


16 


THE    ELECTROMETALLURGY    OF    STEEL 


novel  feature  of  the  furnace  construction  enables  the  furnace 
body  to  be  tilted  for  pouring  without  movement  of  the  iron  core 
and  upper  set  of  primary  coils ;  the  lower  set,  like  that  in  the 
Frick  furnace,  is  attached  to  the  furnace  bottom  and  moves 
with  it. 

The  development  of  the  induction  furnace  shows  that 
considerable  inventive  skill  has  been  concentrated  on  attempts 
to  improve  its  electrical  features,  and  to  render  it  a  useful  ap- 


FIG.  16. 

pliance  in  the  hands  of  the  steel-maker.  Unfortunately  the 
fundamental  elements  of  an  induction  furnace  are  adverse 
to  the  flexibility  and  economy  of  operation  which  are  essential 
to  the  production  of  electric  steel  as  a  commercial  commodity. 
It  is  therefore  not  surprising  that  arc  furnaces  by  their  simul- 
taneous development  have  practically  supplanted  the  induction 
furnace  for  the  melting  and  refining  of  steel.  The  induction 
furnaces  mentioned  in  this  survey  are  only  the  better-known 
types  that  have  been  used  commercially. 

On  returning  to  the  history  of  arc  furnace  development  a 


HISTOKICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES 


17 


brief  description  of  each  type  will  be  sufficient  to  indicate  the 
material  points  of  difference,  a  detailed  description  of  those 
types  now  in  extensive  use  being  given  later. 

Following  the  publication  of  Sir  William  Siemens'  work 
with  arc  furnaces,  sixteen  years  elapsed  before  the  problem  of 
melting  steel  by  arc  heating  was  again  attacked.  In  February, 
1899,  Stassano  made  known  the  results  of  experiments  on  the 
reduction  of  iron  ores  by  electro-thermic  means  conducted  during 


the  preceding  year.  His  experiments  were  confined  to  the  re- 
duction of  ore  to  metal,  which  he  proposed  to  further  refine  in 
the  same  furnace  for  removal  of  carbon  when  necessary. 

His  first  furnace,  built  in  1898  (Fig.  17),  clearly  resembles 
that  type  of  Siemens'  electric  crucible  furnace  which  embodied 
the  principle  of  indirect  arc  heating.  The  furnace  was  built  by 
Stassano  in  Kome  and  used  solely  for  experiments  on  the  re- 
duction of  iron  ore.  A  mixed  charge  consisting  of  briquettes 
was  supported  in  the  basic  lined  shaft  A  by  an  iron  grating 
fixed  8  inches  above  the  arc  zone.  The  reduced  metal  trickled 


18 


THE    ELECTRO-METALLURGY    OF    STEEL 


through  the  unreduced  charge  into  the  crucible  C,  from  whence 
it  was  tapped  at  intervals.  The  slag,  however,  was  not  suffi- 
ciently fusible,  and  remained  as  a  solid  arch  above  the  arc  zone, 
preventing  further  reduction  of  the  charge  above. 

This  serious  difficulty  led  to  a  modified  form  (Fig.  18)  in 
which  the  mixed  charge  was  introduced  below  the  arc  zone 
instead  of  above  it,  and  in  this  way  the  difficulties  of  maintaining 

a  continuous  reaction  were  over- 
come. It  was  found,  however, 
that  during  the  first  stages  of 
reduction  the  power  required 
was  much  greater  than  during 
the  final  stages  when  the  bulk 
of  the  metal  and  slag  had  been 
fused  and  melted.  Owing  to 
the  impossibility  of  reducing  the 
load  without  forming  a  long  and 
unstable  arc,  Stassano  was  later 
forced  to  modify  this  furnace  by 
using  two  or  three  pairs  of  elec- 
trodes in  place  of  one  pair  (Fig. 
19)  ;  the  furnace  load  could  then 
be  varied  within  certain  limits 
by  extinguishing  one  or  more 
arcs.  The  reduction  from  full 
to  low  load  was  at  that  time 
objectionable  to  the  generating 
station,  and  it  was  then  suggested 
that,  by  the  use  of  two  furnaces 
always  operating  at  different 
FIG.  18.  stages  of  the  reduction  process, 

the  mean  load  taken  might  approach  a  more  constant  figure. 
The  plurality  of  electrodes  and  the  excessive  difficulty  of  operat- 
ing the  two  furnaces  so  as  to  require  at  all  times  a  constant 
generating  station  load  soon  showed  the  impracticability  of  this 
arrangement. 

Stassano  then  argued  that,  if  the  period  when  the  minimum 
power  is  required  could  be  shortened  by  rapid  treatment  and 
removal  of  the  metallic  mass  during  the  reducing  fusion  of  the 


HISTORICAL   DEVELOPMENT    OF    ELECTRIC    FURNACES 


19 


charge,  the  furnace  might  be  continuously  operated  at  a  higher 
and  more  uniform  load.  He  had  also  observed  that  increased 
economy  was  gained,  both  in  time  and  fuel,  by  using  a  furnace 
with  a  movable  hearth  for  refining  pig-iron,  and  thereupon 
conceived  the  idea  of  embodying  this  principle  in  his  furnace 
to  overcome  his  difficulties. 

This  was  the  origin  of  the  Stassano  furnace  as  later  used  for 
melting  steel  scrap,  and  which  in  its  first  form  was  patented 


FIG.  19. 

in  1902.  In  1903  Dr.  Goldschmidt,  reporting  on  behalf  of  the 
German  Patent  Office,  stated  that  mild  steel  could  be  success- 
fully made  from  pure  ores,  but  that  the  process  was  too  expen- 
sive for  economic  competition. 

Dr.  Haanel's  report  of  1904  contains  information  communi- 
cated by  Stassano  relative  to  a  furnace  of  1000  horse-power 
rating.  The  output  was  given  as  4  to  5  tons  per  day  according 
to  the  quality  of  ore  used.  The  furnace  was  provided  with 


20  THE    ELECTRO-METALLURGY    OF    STEEL 

two  pairs  of  electrodes  in  parallel,  each  pair  supplying  an  arc 
with  2450  amperes  at  150  volts.  Apart  from  the  usual  fettling, 
the  lining  was  estimated  to  last  at  least  forty  days  without 
repair.  Later  the  rotating  furnace  was  further  modified  to 
meet  the  requirements  of  melting  mixed  steel  and  iron  scrap, 
and  thus  the  production  of  mild  steel  or  iron  by  direct  reduction 
of  ore  gave  place  to  simple  melting  and  refining. 

While  Stassano  was  endeavouring  to  apply  electric  heating 
to  the  more  primitive  method  of  steel-making  by  direct  reduc- 
tion from  ore,  Dr.  Heroult,  in  the  year  1899,  began  his  extensive 
work  on  the  production  of  steel  from  pig-iron,  and  later  from 
common  scrap-iron  and  steel.  He  saw  more  prospect  of  success 
by  following  the  lines  of  a  well-established  process  in  general 
use,  which  consisted  of  refining  pig-iron  and  scrap  in  the  basic 
open  hearth  furnace,  and  succeeded  in  solving  the  problem  of 
carbon  contamination  by  interposing  a  layer  of  slag  between 
the  bath  of  metal  and  the  electrodes,  and  further  by  the  use 
of  a  non-carbonaceous  lining.  This  slag  covering,  which  might 
be  varied  in  composition,  had  the  added  advantage  of  effecting 
removal  of  injurious  impurities  from  the  bath.  The  develop- 
ment of  the  series  arc  design  was  both  interesting  and  logical, 
and  entirely  fulfilled  the  expectations  of  the  inventor. 

In  the  year  1899  Heroult  was  using  a  single  electrode 
furnace,  resembling  in  principle  the  direct  arc  crucible  furnace 
of  Siemens,  for  the  manufacture  of  ferro-alloys.  The  furnace, 
lined  throughout  with  carbon,  was  supplied  with  current  by 
two  electrodes,  one  embedded  in  the  bottom  and  the  other 
hanging  vertically  so  as  to  complete  the  circuit  through  the 
conductive  charge.  The  carbon-lined  bottom  was  satisfactory 
for  the  production  of  high  carbon  ferro-alloys,  but  with  a  grow- 
ing demand  for  a  low  carbon  ferro-chrome  it  had  to  be 
abandoned.  As  a  substitute  Heroult  first  employed  a  bottom 
composed  of  chromite  ore,  surrounding  a  single  carbon  pole  placed 
centrally.  He  expected  that  the  carbon  would  eventually  be 
consumed,  and  replaced  by  metal  to  a  depth  where  the  tempera- 
ture was  sufficiently  low  to  prevent  further  absorption.  This 
bottom  carbon  electrode  was,  however,  found  unsatisfactory, 
and  the  logical  conclusion  of  supplying  power  to  the  upper 
portion  of  the  conductive  charge  followed,  this  being  accom- 


HISTORICAL   DEVELOPMENT    OF   ELECTRIC   FURNACES          21 

plished  by  substituting  another  vertical  top  electrode  for  the 
one  embedded  in  the  bottom.  In  this  way  the  Heroult  steel 
furnace  provided  with  two  hanging  electrodes  and  two  arcs  in 
series  originated. 

The  principle,  then,  of  direct  arc  heating  as  first  used  by 
Siemens  is  followed,  but  with  the  one  all-important  difference 
that  the  electrodes  carrying  current  to  the  furnace  both  enter 
from  above  the  charge  and  so  eliminate  all  electric  connections 
made  with  the  furnace  hearth.  Finding  the  new  type  of  fur- 
nace successful  for  melting  low  carbon  alloys  having  a  high 
melting-point,  Heroult  then  turned  to  the  problem  of  melting 
and  refining  steel. 

The  furnace  lining  could  be  made  similar  to  that  commonly 
used  in  the  Siemens-Martin  furnace,  so  that  there  seemed  little 
reason  why,  given  sufficient  temperature,  the  same  metallurgical 
results  should  not  be  achieved ;  not  only  was  this  the  case  but 
it  was  soon  found  that  this  form  of  furnace  possessed  important 
features  which  permitted  refining  operations  to  be  carried  far 
beyond  anything  hitherto  possible  in  the  open-hearth  furnace. 

Up  to  the  year  1904  nine  distinct  patents,  covering  novelties 
of  furnace  design  and  metallurgical  operations  peculiar  to  the 
electric  furnace,  were  issued  in  different  countries  throughout 
the  world. 

To  show  how  thoroughly  the  investigations  were  carried 
out,  and  how  far  the  value  of  the  hitherto  unknown  chemical 
reactions  was  understood  and  appreciated,  it  is  only  necessary 
to  quote  the  titles  of  the  French  patents  enumerated  below : — 

I.  No.  298,656.     Nineteen  patents  issued.     "Improvements 
in  electric  furnaces,  with  a  view  to  obtaining  soft  metals  and 
other  materials,  in  which  it  is  necessary  to  prevent  contamina- 
tion with  carbon  from  the  electrodes.". 

II.  No.  305,317.     Four  patents  issued.     "  Process  and  ap- 
paratus for  the  electric  manufacture  of  wrought-iron,  steel  and 
cast-iron  by  electric  heating. " 

III.  No.  305,373.     Patent  rights  granted  in  eleven  countries. 
"  Process  and  apparatus  to  make  use  of  the  waste  heat  resulting 
from  the  manufacture  of  pig-iron." 

IV.  No.  307,739.     Thirteen  patents  issued.     "  Tilting  elec- 
tric furnace." 


22  THE   ELECTROMETALLURGY   OF   STEEL 

V.  No.  318,638.     Thirteen  patents  issued.     "  Electric   fur- 
nace with  movable  electrodes." 

VI.  No.    320,682.      Nine   patents   issued.      "Process   for 
deoxidising  and  carburising  liquid  steel." 

VII.  No.    328,350.     Six   patents   issued.     "  Improvements 
in  the  production   of   iron   and    steel   by  electro-metallurgical 
means." 

VIII.  No.  336,705.     Patent  issued  in  France  only.     "Pro- 
cess for  deoxidising  and  desulphurising  steel." 

IX.  No.  356,714.     Patent  of  addition.     "  Improvements  in 
the  production  of  iron  and  steel  by  electro-metallurgical  means." 

The  first  patent  was  applied  for  in  France  in  March,  1900, 
and  covered  the  use  of  a  rectangular  open-top  furnace,  provided 
with  two  or  more  vertical  electrodes  suspended  above  it  and 
connected  in  series.  Particular  mention  is  made  of  two  volt- 
meter circuits,  connected  between  the  electrodes  and  the  metal 
in  the  crucible  for  the  purpose  of  regulating  the  voltage  of  each 
arc,  and  so  dividing  the  load  equally  between  them.  The  elec- 
trodes were  capable  of  being  raised  or  lowered,  and  the  use  of 
single-phase  and  polyphase  alternating  currents  provided  for. 
The  crucible  could  be  lined  with  any  known  refractory  material, 
and  this,  together  with  the  use  of  a  slag  covering,  enabled  metals 
to  be  melted  without  the  usual  contamination  with  carbon  from 
the  bottom  or  from  the  electrodes.  Heroult's  first  furnace  was 
fixed,  as  is  shown  in  Fig.  20,1  and  further  comment  is  unneces- 
sary beyond  drawing  attention  to  the  power  and  voltmeter 
circuits. 

In  the  second  patent2  Heroult  describes  the  application  of 
his  first  furnace  for  the  production  of  wrought-iron  and  steel, 
and  outlines  the  process  to  be  used.  The  manufacture  of  steel 
was  conducted  in  successive  operations  in  the  same  furnace. 
Iron  ore  was  first  reduced  to  pig-iron,  which,  after  removal  of 
the  bulk  of  the  slag  through  the  upper  tap  hole,  could  be  further 
refined  by  the  addition  of  ore.  The  "  refining  "  action,  or  boil- 
ing down,  could  be  stopped  at  any  moment  when  the  desired 
percentage  of  carbon  had  been  arrived  at,  this  being  determined 
by  constant  sampling  and  fracture  of  bath  tests.  By  making 

1  British  Patent  Specification,  No.  16,293,  A.D.  1900. 

2  Ibid.,  No.  14,486,  A.T>.  1901. 


HISTOEICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES         23 

the  slag  thoroughly  basic  a  "  purifying  "  action,  resulting  in 
the  removal  of  phosphorus,  was  made  feasible.  According  to 
Heroult  the  "purifying"  and  "refining"  actions — using  his 
own  terms — could  proceed  simultaneously  or  otherwise  as  re- 
quired, and  the  process  could  be  conducted  in  the  same  manner 
whether  the  carburised  metal  was  charged  molten  or  in  solid 
pieces.  In  the  patent  specification  a  disclaimer  is  made  point- 
ing out  the  essential  differences  between  the  crucible  furnace 
of  Siemens  and  the  furnace  in  question.  The  furnace  was 
substantially  the  same  as  that  shown  in  Fig.  20,  but  was  pro- 


vided with  a  movable  roof  which  confined  the  heat  to  the 
interior  and  could  be  readily  changed.  The  next  patent  may 
be  passed  over  as  it  does  not  bear  directly  upon  the  manufac- 
ture of  steel. 

The  tilting  furnace  l  was  the  next  development  (Fig.  21). 
The  construction  shown  embodied  all  the  essential  features 
of  the  modern  furnace,  and  was  designed  to  facilitate  the 
refining  and  purifying  processes  as  previously  conducted  in 
the  fixed  furnace.  Other  novelties  were  at  the  same  time 
introduced,  including  the  method  of  conveying  current  to  the 

1  British  Patent  Speciacation,  No.  14,643,  A.D.  1901. 


24  THE    ELECTRO-METALLURGY    OF    STEEL 

movable  electrodes,  and  the  use  of  an  air  blast  to  hasten  the 
refining  action. 

A  method  of  deoxidising  and  carburising  steel  produced  by 
the  refining  and  purifying  processes  is  described  in  a  later 
patent.1  The  method  consisted  of  throwing  on  to  the  bath 
briquettes  composed  of  a  carbonaceous  material  agglomerated 
with  iron  and  steel  filings  or  turnings,  which,  by  their  density, 
forced  the  briquettes  through  the  slag  into  contact  with  the 
steel,  so  that  the  carbon  might  be  more  easily  absorbed.  The 
use  of  this  material  has  not  met  with  much  favour,  as  the 


FIG.  21. 

carbon  absorption  was  too  low  and  erratic,  any  variation  in  the 
size  of  the  pieces,  slag  condition,  and  temperature  of  the  bath 
influencing  the  degree  of  absorption.  It  is  doubtful  also  whether 
any  deoxidisation  was  effected  except  by  the  subsequent  indirect 
action  of  any  carbon  entrapped  in  the  slag. 

It  was  later  realised  that  the  process  of  decarburising  pig- 
iron  by  electrical  energy  was  not  economical,  and  it  was  then 
proposed  to  transfer  liquid  steel,  which  had  been  either  blown 
in  a  Bessemer  converter  or  boiled  down  in  an  open  hearth 
furnace,  to  the  electric  furnace  for  the  final  conversion  to  finished 

1  British  Patent  Specification,  No.  6950,  A.I>.  1903. 


Fm.  22. 


[To  face  p.  25. 


HISTORICAL   DEVELOPMENT    OF   ELECTRIC   FURNACES          25 

steel.     This  method,  which  amounts  to  a  Duplex  process,  is 
described  in  a  further  patent.1 

Lastly,  the  process  for  deoxidising  and  desulphurising  steel  as 
is  now  known  was  patented  in  France  in  November,  1903,  so 
that,  in  rather  more  than  three  years,  all  those  metallurgical 
treatments  essential  to  the  production  of  high  quality  electric 
steel  in  arc  furnaces  had  been  discovered  and  made  use  of  com- 
mercially. These  treatments  may  be  briefly  summarised  as 
follows : — 

I.  2nd  Patent. — Melting  cold  cast-iron  and  steel  scrap  by 
arc  heating. 

II.  1st  Patent. — Kegulation  of  load. 

III.  2nd   Patent. — Oxidation    of    carbon    and    removal    of 
phosphorus  and  other  impurities  by  a  basic  slag. 

IV.  2nd  Patent. — Kemoval  of  the  foul  slag. 

V.  6th    Patent. — Subsequent    carburising    of    the"  bath   if 
required. 

VI.  8th  Patent. — Deoxidation  and  desulphurising  by  means 
of  a  highly  basic  slag  free  from  oxides. 

VII.  6th  Patent. — Addition  of  alloys  to  the  bath  without  loss 
prior  to  casting. 

VIII.  7th  and  9th  Patents.— Kenning  liquid  steel. 

In  1904  the  Canadian  Government  Commission  visited  La 
Praz  to  investigate  the  Heroult  process  then  being  practised  in 
the  first  tilting  furnace  built  (Fig.  22). 

The  furnace  was  connected  to  a  single-phase  alternator 
coupled  direct  to  a  water  turbine.  At  normal  speed  the  voltage 
was  110  and  the  frequency  33,  the  power  at  full  load  being  about 
360  kw.  and  generally  much  less  during  the  melting  down 
operation.  The  power  consumption,  in  the  case  of  those  heats 
witnessed  by  the  commission,  averaged  1100  units  per  ton  of 
ingots  both  for  dead  soft  and  high  carbon  steels.  The  miscel- 
laneous scrap  used  was  very  impure,  containing  0*22  per  cent, 
phosphorus,  and  the  period  of  refining  was  consequently  pro- 
longed. The  radiation  loss  of  the  furnace  was  also  very  high  in 
relation  to  the  total  available  power,  so  that  the  high  power  con- 
sumption was  to  be  expected. 

1  British  Patent  Specification,  No.  7027,  A.D,  X903, 


26  THE    ELECTRO-METALLUKGY    OF    STEEL 

The  single-phase  rectangular  furnace  was  followed  later  by 
the  three-phase  furnace  of  circular  form,  and  with  this  modifi- 
cation the  ultimate  development  of  the  Heroult  furnace  was 
reached,  as  characterised  by  its  essential  features. 

At  the  same  time  that  Heroult  was  working  on  the  problem 
of  making  low  carbon  alloys,  another  investigator,  Charles  A. 
Keller,  was  similarly  engaged.  The  early  work  of  the  latter, 
which  led  to  the  issue  of  a  patent 1  in  France  in  1900,  was  briefly 
described  by  the  inventor  himself  in  a  paper  delivered  before 
the  American  Electro-Chemical  Society  in  1909. 2  From  the 
description,  the  furnace  closely  resembled  that  patented  by 
Heroult  in  March  of  the  same  year  embodying  :— 

I.  The  use  of  two  separate  heating  zones  at  the  upper  sur- 
face of  the  conductive  charge. 

II.  Voltage  regulation  of  each  electrode. 

III.  Arrangement  of  the  electrodes  in  series. 

IV.  Construction  of  the  furnace  to  obtain  products  by  the 
tapping  method. 

Keller  experimented  with  a  furnace  of  15  cwts.  capacity  up 
to  the  year  1902  at  the  Kerrouse  Works  (Morhiban,  France), 
more  especially  for  the  manufacture  of  steel  from  cold  scrap. 
The  ingots  were  tested  at  different  works  near  St.  Etienne  and, 
following  the  favourable  results  obtained,  a  larger  furnace  of 
2^  tons  capacity  was  installed  at  the  works  of  the  Societe  des 
Etablissements  Keller  Leleux  at  Livet  (France).  This  furnace 
was  likewise  provided  with  two  electrodes  and  was  experimented 
with  for  three  years,  during  which  time  the  metallurgical 
problems  and  products  made  were  investigated  by  J.  Holtzer  & 
Co.,  of  the  Unieux  Steel  Wo  rks^  (Loire).  Prompted  by  further 
success,  it  was  decided  to  install  a  furnace  of  8  to  10  tons  capacity 
at  the  latter  works,  several  modifications  of  interest  being 
incorporated  in  the  design.  The  furnace  body  (Fig.  23), 
consisting  of  a  steel  shell  lined  with  a  suitable  refractory  material 
and  mounted  for  tilting  on  rollers,  was  entirely  independent  of 
the  electrode  carrying  gear.  A  novel  feature  was  introduced  by 
interleaving  the  bus  bars  and  bringing  them  above  and  then 
over  the  furnace  body,  where  they  terminated  in  a  special 

1  French  Patent,  No.  300,630,  23/5/1900. 

2  "Am.  Electro-Chem.  Society,"  Vol.  XV,  1909. 


HISTORICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES 


27 


distribution  block.  The  bus  bars  of  different  polarity  were 
there  divided,  and  each  group  provided  with  two  separate  sets  of 
lugs,  so  arranged  that  the  flexible  connection  to  the  electrode 


holders  might  be  rapidly  and  securely  made  when  changing 
electrodes.  The  latter  were  suspended  from  jib  cranes,  which 
swung  into  a  position  over-hanging  the  furnace  roof.  The 


28  THE    ELECTRO-METALLURGY   OF    STEEL 

furnace  operated  on  a  single-phase  supply,  and  the  load  circuit 
contained  two  pairs  of  arcs  in  series  in  place  of  two  single  arcs, 
as  in  the  Heroult  furnace,  the  line  voltage  being  equally  divided 
between  each  pair  of  arcs.  The  two  electrodes  of  each  group 
were  capable  of  individual  movement  and  electrically  connected 
in  parallel.  There  were,  then,  four  distinct  arcs,  one  pair  of 
which  was  in  series  with  the  other  pair,  whilst  each  pair  con- 
sisted of  two  arcs  in  parallel.  The  regulation  was  necessarily 
complicated,  any  inequality  of  current  flowing  through  the  two 
electrodes  of  either  pair  being  adjusted  by  the  simultaneous 
lowering  of  one  and  raising  of  the  other ;  balance  of  voltage 
between  each  pair  and  the  bath  was  likewise  effected  by  raising 
one  pair  and  lowering  the  other.  Other  combinations  of  move- 
ments were  also  made  possible  by  a  system  of  valves  operating 
hydraulic  cylinders.  The  furnace  at  Unieux  was  used  for  refin- 
ing open  hearth  furnace  steel  already  low  in  phosphorus  but 
high  in  sulphur.  The  power  consumption  per  ton  of  steel 
averaged  275  kw.-hours,  a  load  of  750  kw.  being  taken  for  about 
2|  hours.  The  experimental  work  at  Livet  done  prior  to  the 
erection  of  this  large  furnace  at  Unieux  was  concluded  in  1905, 
and  in  the  meantime  led  to  the  issue  of  several  French  patents. 

I.  French  Patent,  No.  300,630.     23/5/1900.     "Electric  fur- 
nace improvements." 

II.  French  Patent,  No.  322,700.    2/7/02.     "  Process  for  melt- 
ing and  refining  metals  and  other  substances  electrically." 

This  invention  was  also  patented  in  England l  and  described 
as  follows : — 

"  Metals  or  other  substances  are  electrically  heated  for  re- 
fining or  other  purposes  by  current  passing  between  electrodes 
E  (Fig.  24)  dipping  into  the  materials.  Preferably  several 
electrodes  are  used,  so  that  one  may  be  changed  without  stopping 
the  operation.  The  heating  may  be  effected  in  a  furnace  G, 
arranged  to  receive  the  product  of  a  cupola  furnace  F  :  or  in  an 
ordinary  foundry  ladle,  which  is  carried  on  a  movable  truck  so 
that  it  may  be  charged  from  one  or  more  furnaces  and  then 
placed  under  vertically  movable  electrodes.  Metal  may  thus 
be  collected  and  kept  hot  for  casting,  and  other  materials  may 

1  British  Patent  Speciacation,  No.  15,271,  1902. 


HISTORICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES         29 

be  added  to  effect  refining ;  waste  iron  or  steel  may  be  added  and 
steel  produced." 

The  specification  clearly  states  that  the  electrodes  dip  into 
the  "material"  (slag  in  the  case  of  steel-making),  and  are  ar- 
ranged in  two  groups  of  opposite  polarity,  each  group  being 
built  up  of  two  or  more  electrodes  separately  movable  and  con- 
nected in  parallel. 


FIG.  24. 

III.  No.  329,013.     2/2/03.     "  Improvements  in  the  process 
of  electric  melting  and  refining." 

The  feature  of  this  patent  lies  in  the  use  of  free  arcs  formed 
between  the  electrodes  and  the  slag,  as  opposed  to  the  electrodes 
dipping  into  the  slag  and  heating  purely  by  resistance.  In  this 
way  alone  can  an  oxidised  slag  be  maintained.  Oxidising  and 
dephosphorising  could  be  performed  under  a  suitable  slag  and 
then  followed  by  a  refining  period  under  a  different  slag. 

IV.  No.    387,461.      6/5/07.      "Process   for    carburising    a 
liquid  metal." 


30 


THE   ELECTRO-METALLURGY   OF    STEEL 


Composite  blocks  are  made  by  pouring  a  metal  into  a  mould 
previously  filled  with  small  pieces  of  broken  carbon.  Such 
blocks  when  thrown  on  a  bath  of  metal  will  sink  by  their  weight, 
into  close  contact  with  the  metal  to  be 


and  bring  the  carbon 


FIG.  25. 

carburised.  In  the  case  of  steel,  cast-iron  is  the  metal  used 
for  making  the  blocks. 

V.  No.  393,740.  4/11/07.  "  System  of  conducting  hearth 
for  electric  furnaces  "  (Fig.  25). x 

This  invention  relates  to  the  use  of  a  conducting  hearth, 
composed  of  a  refractory  material  suitable  for  steel  manufacture 

1  "  Am.  Electro-Chem.  Society,"  Vol.  XV,  p.  98. 


HISTOKICAL   DEVELOPMENT   OF   ELECTKIC   FURNACES         81 

• 

and  capable  of  carrying  the  full  load  current.  It  will  be  re- 
membered that  a  carbon  bottom,  serving  as  one  electrode,  had 
been  used  prior  to  1900  for  the  manufacture  of  ferro-alloys,  but 
was  abandoned  by  Heroult  for  the  reason  of  carbon  contamina- 
tion in  cases  where  low  carbon  alloys  or  steel  were  to  be  made. 
On  returning  to  the  use  of  a  conducting  hearth,  it  was 
necessary  to  employ  a  material  free  from  carbon,  which  was  at 
the  same  time  sufficiently  conductive  to  carry  the  full  current 
when  cold.  For  this  purpose  Keller  used  a  composite  bottom, 
consisting  of  a  number  of  iron  rods  connected  to  a  fixed  bottom 
plate  and  embedded  in  magnesite  or  dolomite,  mixed  with  a  tar 
or  pitch  binder  rammed  in  to  form  a  solid  plug.  The  metallic 
portion  of  the  hearth  was  relied  upon  to  complete  the  circuit 
when  cold.  The  distribution  of  the  current  was  equalised  over 
the  entire  section  of  the  hearth,  and  in  this  respect  differs  from 
the  metallic  bottom  electrodes  used  by  Girod  about  three  years 
earlier. 

VI.  No.  400,461.     6/6/08.     "  Process  for  the  exact  carburis- 
ing  of  steel." 

This  method  consists  in  the  immersion  of  a  carbon  block 
into  the  bath  of  steel  in  such  a  manner  that  the  exact  loss  in 
weight  by  absorption  in  the  bath  is  accurately  recorded  as 
carburising  proceeds. 

VII.  No.  399,643.     27/4/08.    "  Eadiating  distribution  of  con- 
ductors for  multiple  electrode  furnaces  "  (Fig.  26). l 

This  patent  refers  to  a  method  of  interleaving  the  bus  bars 
of  opposite  polarity  and  bringing  them  to  a  central  point  of  dis- 
tribution, whence  the  current  flows  through  suitable  radiating 
connecting  lugs  to  the  four  electrodes,  as  previously  described 
for  the  8  to  10  ton  furnace  built  at  the  Unieux  works. 

VIII.  No.    400,655.     15/6/08.     "  System   of  regulating  the 
circuits  supplying  multiple  electrode  furnaces." 

This  method  was  embodied  in  the  four-electrode  furnace  as 
described,  and  consists  of  voltage  and  current  regulation  of  the 
electrodes  in  series  and  parallel  respectively. 

IX.  No.   14,728/393,740.      21/11/10.      Patent   of    Addition. 
"  System  of  a  conducting  hearth  for  electric  furnaces." 

1  "  Am.  Electro-Chem.  Society,"  Vol.  XV,  p.  114. 


32 


THE    ELECTRO-METALLURGY   OF    STEEL 


In  this  case,  the  composite  bottom  previously  described  is 
replaced  by  ramming  the  entire  hearth  with  a  mixture  of 
magnesite  and  iron  filings,  bound  together  by  tar  or  pitch.  For 
operation  on  a  three-phase  supply,  Keller  proposed  to  connect 
the  three  electrode  circuits  either  in  delta  or  star,  when,  in  the 
latter  case,  the  conducting  hearth  is  made  the  neutral  and  con- 
nected to  the  star  point  of  the  transformer  secondary  circuits. 


FIG.  26. 


In  a  general  survey  of  Keller's  inventions  it  will  be  seen  that 
for  seven  years  his  efforts  were  restricted  to  furnaces  employing 
suspended  electrodes  operating  in  series,  the  bottom  electrode 
or  conducting  hearth  being  only  adopted  towards  the  end  of 
1907.  Paul  Girod  had  already  in  1905  utilised  a,  modification 
of  the  old  carbon  bottom  for  melting  steel,  and  his  contribution 
to  the  general  study  of  electric  steel-making  must  next  be 
mentioned. 


HISTORICAL  DEVELOPMENT   OF  ELECTRIC   FURNACES         33 

Girod  had  been  engaged  for  many  years  in  the  use  of  electric 
furnaces  for  the  reduction  of  ores  before  turning  to  steel  melt- 
ing. His  first  invention  is  particularly  interesting,  as  it  was 
the  first  application  of  Siemens'  original  principle  of  direct  arc 
heating  to  the  commercial  manufacture  of  steel.  Heroult  and 
Keller  had  overcome  the  difficulties  due  to  a  carbon  electrode 
bottom  by  eliminating  it,  whereas  Girod  preferred  to  construct 
a  conductive  hearth  which  would  prevent  all  possibility  of 
carbon  contamination.  The  successive  steps  and  improve- 
ments, evolved  from  the  time  of  his  first  studying  the  subject, 
will  be  again  best  followed  by  enumeration  of  the  several 
patents  issued  in  France. 

I.  French  Patent,  No.  350,524-  4/1/05.  "Electric 
furnaces." 

The  nature  of  this  invention  is  concisely  described  in  the 
resume  of  the  specification  of  which  the  following  is  a  trans- 
lation :  "  An  electric  furnace  in  which  one  of  the  poles  is  formed 
by  one  or  several  graphite  electrodes,  moved  mechanically  or  by 
hand,  the  other  pole  being  constituted  of  several  electrodes 
buried  in  the  furnace  lining,  and  situated  at  such  distance  from 
the  hearth,  or  cooled  artificially  in  such  fashion,  that  at  their 
extremities  a  certain  quantity  of  fused  material  is  chilled  and 
forms  upon  them  a  solid  protective  layer  which  prevents  con- 
tact of  these  electrodes  with  the  metal  being  manufactured. 
Contact  between  the  latter  and  the  movable  electrode  is  avoided 
by  the  fact  that  the  said  electrode  dips  only  into  the  layer  of 
slag  which  covers  the  metal  on  melting." 

In  the  subject-matter  of  the  specification  further  details  are 
given  of  the  manner  of  constructing  the  bottom  conductors 
(Fig.  27). 

The  actual  poles,  which  may  be  of  metal  or  graphite,  are 
usually  water-cooled,  and,  if  insulated,  may  be  connected  in 
parallel  or  in  series  to  the  bus  bars.  In  the  case  of  graphite, 
the  poles  are  covered  at  their  upper  extremities  with  a  metallic 
pole  piece,  which  either  partly  melts  or  becomes  enlarged  during 
operation  of  the  furnace. 

When  the  furnace  is  circular  in  shape  the  poles  give  place 
to  an  annular  channel,  which  is  filled  with  cast-iron  and  similarly 


34 


THE    ELECTRO-METALLURGY   OF   STEEL 


water-cooled ;   this  particular  construction  was  especially  sug- 
gested for  making  steel  from  pig-iron  (Fig.  28).     The  furnace 


Fm.  27. 


w//////////// 


FIG,  28. 


can  be  either  fixed  or  constructed  for  tilting,  and  can  operate  on 
either  continuous  or  alternating  current  supplies,     "With  proper 


HISTORICAL   DEVELOPMENT    OF   ELECTEIC   FURNACES          35 

choice  of  voltage  the  electrodes  will  form  arcs  above  the  slag, 
when  the  known  processes  of  decarburising,  dephosphorising  and 
desulphurising  can  be  conducted.  Preference  is  given  to  the  use 
of  a  non-conducting  material  surrounding  the  pole  pieces  or 
conductors.  In  the  single-phase  design  the  current  from  the 
upper  central  electrode  is  forced  to  take  a  diagonal  path  to 
the  two  lower  electrodes,  causing  circulation  of  the  metal. 

In  1902  Heroult  filed  a  patent  application  in  Belgium  which 
covered  the  use  of  carbon-bottom  electrodes  with  metallic  end 
pieces  to  prevent  carbon  contamination  of  the  bath.  Girod, 
therefore,  was  not  the  first  to  conceive  the  idea  of  such 
bottom  electrodes,  but  was  the  first  to  put  it  to  commercial 
use. 

II.  French  Patent,  No.  350,802.    16/1/05.    "  Tilting  furnace 
with  conducting  hearth." 

In  his  first  patent,  Girod  suggested  that  his  furnace  might 
be  constructed  to  tilt.  In  this  patent  a  specific  method  of 
tilting  the  furnace  on  trunnions  is  described,  and  the  provision 
of  two  tapholes  at  different  levels  is  mentioned. 

III.  Patent  of  Addition,  No.  4829/350,524.    4/1/05.    "Electric 
furnaces." 

In  place  of  the  annular  conducting  ring  previously  patented, 
the  conducting  poles  may  be  formed  by  leaving  channels  in  the 
furnace  lining  extending  upwards  from  the  shell  plate  to  the  level 
of  the  working  bottom.  These  channels  are  filled  up  with 
solid  lumps  of  metal,  which,  during  a  preliminary  operation  of 
the  furnace,  become  fritted  together  to  form  a  solid  pole. 

IV.  No.  388,614.      4/6/07.     "  Process  for  the  manufacture 
of  iron  and  steel  in  the  electric  furnace." 

This  process  requires  the  use  of  at  least  two  electric  furnaces, 
one  being  employed  to  melt  and  refine  cold  scrap,  the  other  of 
much  smaller  capacity  being  used  solely  to  finish  the  oxidised 
and  dephosphorised  steel  from  the  former.  The  large  melting 
furnace  is  never  emptied  during  continuous  operation,  sufficient 
steel  being  retained  to  take  up  an  additional  charge  of  cold 
scrap  equal  to  the  quantity  transferred  to  the  smaller  furnace 
for  finishing.  This  process  saves  the  time  and  labour  of  skim- 
ming off  the  oxidised  slag  prior  to  finishing,  and,  at  the  same 


36  THE   ELECTRO-METALLURGY   OF   STEEL 

time,  obviates  the  loss  of  heat  when  charging  cold  scrap  into  an 
empty  furnace. 

V.  No.  402,758.     6/5/09.     "Process  of  refining  liquid  steel 
from  furnaces  other  than  electric." 

Purposely  under-oxidised  steel  from  an  open-hearth  furnace 
or  converter  is  transferred  to  the  electric  furnace,  and  there 
cooled  down  to  promote  evolution  of  dissolved  gases  (hydrogen 
and  nitrogen).  This  is  followed  by  again  raising  the  temperature, 
when  a  further  elimination  of  C,  Si,  Mn  and  P  is  effected  in 
the  ordinary  manner  under  a  basic  slag. 

VI.  No.  11924/402,758.     7/12/09.     "  Patent  of  addition." 
The  process  is  similar  to  the  above,  but  is  carried  further  by 

successive  raising  and  lowering  of  temperature,  the  evolution  of 
dissolved  gases  being  assisted  by  a  small  addition  of  carbon 
before  reducing  the  temperature.  This  addition  is  also  made 
after  dephosphorising. 

VII.  No.  12771/402,758.     15/6/10.     "Patent  of  addition." 
This  patent  has  for  its  object  the  removal  of  dissolved  oxides 

together  with  the  gases.  A  small  quantity  of  C,  Si,  and  Mn  is 
left  in  the  steel  before  transference  to  the  electric  furnace,  and 
on  reduction  of  temperature  reacts  with  the  dissolved  oxides  to 
form  fusible  insoluble  silicates,  which  readily  fuse  and  rise  up- 
wards through  the  bath.  C,  Si,  and  Mn  may  be  actually  added 
to  promote  this  reaction. 

VIII.  No.  416,927.     9/6/10.     "  Method  of  arranging  electric 
furnaces  for  three-phase  working." 

This  patent  refers  to  a  special  method  of  connecting  the 
three  upper  electrodes  in  star  fashion,  the  bottom  electrode 
being  connected  to  the  neutral  point  of  the  secondary  circuits. 
One  phase  of  the  star  connection  is  reversed,  which  compels  the 
major  portion  of  the  current  to  flow  through  the  bottom  elec- 
trode, while  the  load  is  nearly  equally  balanced  on  the  supply 
phases  (Fig.  61). 

IX.  No.  422,717.     26/8/10.     "  Method  of  arranging  electric 
furnaces  for  three-phase  working." 

Three  methods  are  given  of  connecting  a  furnace,  provided 
with  two  upper  electrodes  and  a  bottom  electrode,  to  operate 


HISTORICAL   DEVELOPMENT   OF   ELECTEIC   FUENACES         37 

on  a  three-phase  supply  system.     In  each  case  the  low  tension 
supply  to  the  furnace  is  actually  two-phase. 

Up  to  the  year  1905  the  only  direct  arc  furnaces  used  com- 
mercially for  steel-making  belonged  to  that  class  in  which  the 
electrode  circuits  were  independent  of  the  furnace  lining.  The 
advent  of  the  Girod  furnace  in  that  year,  and  the  practical  re- 
sults achieved,  led  others  to  modify  the  hearth  construction  and 
to  adopt  it  for  use  with  different  systems  of  electrical  connec- 
tions. Girod  restricted  the  construction  of  his  conducting 
hearth  to  metallic  or  graphite  poles,  embedded  in  a  material 
which  was  preferably  a  poor  conductor  at  high  temperatures, 
and  therefore  relied  entirely  upon  primary  conductors  for  com- 
pletion of  the  circuit  through  the  charge  and  furnace  hearth. 
Keller  in  1907  modified  the  above  method  by  using  a  composite 
bottom  as  already  described,  and  the  value  of  a  conducting 
material,  consisting  of  magnesite  with  a  carbonaceous  binder, 
was  fully  realised  by  him  although  used  in  conjunction  with 
metallic  rods  as  primary  conductors. 

There  was,  however,  considerable  prejudice  against  the  use 
of  metallic  conductors  embedded  in  a  furnace  bottom  when 
directly  in  contact  with  the  molten  bath,  and  it  was  suggested 
that  considerable  difficulty  might  be  occasioned  in  repairing  a 
bottom  between  heats,  owing  to  partial  emptying  of  the  holes 
which  normally  constituted  the  metallic  electrodes.  This  pre- 
judice was  not  always  borne  out  in  practice.  With  a  view,  no 
doubt,  to  avoid  such  difficulties,  a  furnace  was  constructed  at 
the  Firminy  Steel  Works  in  France  and  a  patent  applied  for 
in  March,  1908.  The  hearth  had  no  metallic  conductors,  and 
was  built  up  of  successive  layers  of  a  refractory  material,  such 
as  magnesite  or  dolomite,  mixed  with  a  carbonaceous  binder 
in  decreasing  quantities  towards  the  upper  surface.  It  was 
claimed  that  a  highly  conducting  hearth  could  be  built  up  in 
this  manner  without  fear  of  carbon  contamination  of  the  bath, 
and  with  a  possibility  of  restarting  a  furnace  when  cold.  This 
type  of  hearth  construction  has  now  been  generally  adopted  in 
preference  to  the  metallic  electrodes  used  by  Girod  and  Keller, 
and  of  modern  furnaces  using  conductive  hearths  the  Electro- 
metals  design  was  the  first  to  gain  a  wide  application. 

The  early  forms  of  single-phase  arc  furnace  usually  required 


38 


THE   ELECTRO-METALLURGY   OF    STEEL 


a  special  generating  plant,  owing  to  the  impossibility  of  trans- 
forming from  either  two-  or  three-phase  supplies  with  static 
transformers,  and  it  was  to  overcome  this  fundamental  objec- 
tion that  the  Electro-metals  design  was  introduced ;  the  fur- 
nace operated  on  a  two-phase  low  tension  supply,  transformed 
from  either  a  two-phase  or  three-phase  high  tension  system. 
The  original  patent l  was  applied  for  in  Sweden  in  August, 
1908,  and  embodied  the  use  of  one  upper  electrode  in  each 
phase  circuit,  which  also  included  a  conductive  hearth  and  a 
common  neutral  return  conductor,  the  latter  being  connected 

from  the  hearth  to  the  neutral 
point  of  the  two  phases  (Fig.  46). 
A  later 2  patent  describes  an 
improved  form  of  rectangular 
tilting  furnace ;  the  electrode 
holders  are  mounted  on  swivel 
brackets,  and  so  can  be  raised 
and  swung  round  clear  of  the 
roof.  The  electrode  regulating 
motors  impart  movement  to  the 
electrode  brackets  through  a 
system  of  screw  feed  and  tele- 
scopic shafting,  and  so  can  be 
situated  away  from  the  furnace 
body.  This  construction  (Fig. 
29)  is  not,  however,  adopted  in 
modern  types. 

In  1911  a  modification  of  the  Electro-metals  furnace  was 
introduced  by  Stobie,3  who  substituted  a  return  conductor  for 
each  phase  in  place  of  the  common  return.  The  two  phases  were 
thus  separated  and  constituted  a  four-wire  two-phase  system  in 
place  of  a  three-wire  neutral  return  system ;  the  bottom  elec- 
trode of  each  phase  was  situated  diagonally  opposite  its  corre- 
sponding upper  electrode  in  place  of  below  it  (Fig.  49).  In 
a  more  recent  furnace  4  of  larger  capacity  the  two-phase  bottom 
electrode  type  gives  place  to  a  four  electrode  three-phase  star- 


FIG.  29. — Electro-metal  tilting 
furnace. 


1  Swedish  Patent,  No.  28,687,  1/8/08. 

2  British  Patent,  No.  12,430,  21/5/09. 
4  Ibid.,  No.  2081,  1912. 


Ubid.,  No.  6741,  1911. 


HISTORICAL   DEVELOPMENT   OF   ELECTRIC   FURNACES          39 

connected  type,  in  which  one  electrode  is  connected  to  the 
star-point  of  the  three-phase  system  and  serves  as  a  return  for 
any  unbalanced  current. 

An  ingenious  application  of  two-phase  current  to  indirect 
arc  furnaces  of  the  Stassano  type  was  introduced  by  Renner- 
felt 1  in  1912  with  the  object  of  reducing  the  excessive  wear  of 
the  roof  and  lining.  In  this  furnace  two  arcs  are  formed  be- 
tween the  horizontal  phase  electrodes  and  a  vertical  neutral, 
and  are  deflected  downwards  by  mutual  repulsion  of  the 
magnetic  fields  set  up. 

The  value  of  small  electric  furnaces  for  the  manufacture  of 
light  castings  had  been  well  proved,  and  other  new  types  were 
introduced  to  meet  the  special  requirements  of  the  foundry. 
The  Snyder  furnace,2  together  with  the  Rennerfelt  furnace  just 
alluded  to,  may  be  regarded  as  falling  in  this  category.  The 
furnace  operates  on  a  single-phase  supply,  and  is  provided 
with  one  top  and  one  bottom  electrode,  the  latter  being  em- 
bedded in  the  hearth  and  in  direct  communication  with  the 
metallic  charge  or  bath.  The  chief  feature  lies  in  the  use  of 
a  high  arc  voltage,  usually  about  110  volts  at  full  load,  which 
produces  a  long  arc  from  which  the  roof  and  upper  lining  of 
the  furnace  is  only  slightly  shaded  by  the  electrode  itself.  By 
using  a  high  voltage  arc  the  current  required  for  a  given  power 
input  is  correspondingly  small,  which  entails  a  saving  in  the 
cost  of  electrical  equipment.  It  is  also  claimed  that  a  charge 
of  steel  scrap  may  be  melted  quicker,  and  this  is  now  generally 
accepted  as  being  true,  provided  the  electrical  equipment  is  at 
the  same  time  designed  to  prevent  heavy  current  fluctuations. 
The  Snyder  furnace,  designed  on  this  principle,  enables  a  very 
steady  load  to  be  maintained  even  when  melting  irregular 
shaped  scrap,  but  this  can  only  be  done  at  the  sacrifice  of 
power  factor  which  is  considerably  lowered  by  increasing  the 
reactance  of  the  circuit.  The  construction  of  the  original 
furnace  is  shown  diagrammatically  by  Fig.  30.  The  original 
specification  lays  stress  on  the  special  construction  of  the  fur- 
nace shell,  which  is  composed  of  a  number  of  laminated  steel 
sheets  to  prevent  the  heating  effect  of  eddy  currents,  and  at 
the  same  time  provides  the  necessary  reactance  of  the  circuit. 

1  British  Patent  Specification,  No.  7367,  1912.         276id.,  No.  25,171,  1913. 


40 


THE   ELECTROMETALLUKGY   OF    STEEL 


In  the  case  of  the  conductive  hearth  furnaces  so  far  con- 
sidered, the  current  flowing  through  the  hearth  has  been  fixed 
as  a  definite  proportion  of  the  total  current  flowing  through  the 
upper  electrode  circuits.  This  has  been  considered  an  objection 
by  some,  and  attempts  have  been  made  to  arrange  the  electrical 
installation  and  the  method  of  furnace  connections  so  as  to 
admit  of  varying  at  will,  within  certain  limits,  the  amount  of 
current  flowing  through  the  conducting  hearth. 

The  first  move  in  this  direction,  was  made  by  Dixon,  whose 

earliest  patent  application  l  was  filed 
in  1914.  The  furnace  was  provided 
with  four  or  six  upper  electrodes 
and  one  bottom  electrode.  By 
means  of  switches  in  the  high 
tension  circuit  the  phase  relations 
of  the  low  tension  transformer 
circuits  could  be  altered  to  vary 
the  proportion  of  the  current  flow- 
ing through  the  conducting  hearth  ; 
the  same  result  could  also  be  ac- 
complished by  reversing  one  or  more 
of  the  transformer  connections, 

which  had  been  suggested  by  Girod  in  1911  in  the  case  of  three- 
phase  supply.  Two  later  patents  2  referred  to  special  arrange- 
ments of  a  two-phase  low  tension  supply  suitably  connected  to 
four  electrodes  so  as  to  render  equal  distribution  of  the  load 
between  the  four  electrodes  possible  by  hand  or  automatic  con- 
trol depending  upon  current  variation  only. 

A  simple  single-phase  indirect-arc  furnace  was  introduced 
in  Italy  in  1914  by  F.  Bassanese  and  has  there  acquired  con- 
siderable popularity.  Certain  unusual  features  were  presented 
by  the  method  of  electrode  adjustment,  but  in  general  appear- 
ance the  furnace  resembles  the  revolving  steel  melting  furnace 
of  Stassano. 

Another  furnace  of  more  simple  design,  but  presenting  the 
same  feature  of  a  variable  bottom  electrode  current  as  first 
conceived  by  Dixon,  was  introduced  by  T.  H.  Watson  &  Co. 

1  British  Specification,  No.  4742,  1914. 

*Ibid.,  No.  111,103,  October,  1916;  No.  111,104,  October,  1916. 


FIG.  30. — Snyder  furnace. 


HISTORICAL   DEVELOPMENT    OF   ELECTRIC   FURNACES         41 

(Sheffield),  and  is  better  known  as  the  Greaves-Etchells  furnace. 
According  to  the  first  patent1  the  furnace  could  operate  on 
either  a  three-phase  or  two-phase  supply  system,  and  further 
by  the  use  of  only  two  upper  electrodes  a  good  balance  of  load 
could  be  obtained  on  the  primary  supply  phases,  where  static 
transformers  are  used.  In  one  case,  when  the  current  through 
the  two  upper  electrodes  is  equal,  the  voltage  in  the  secondary 
windings  connected  to  the  lower  electrode  can  be  varied  to 
produce  the  required  balance.  Further  modifications2  were 
later  made  in  the  use  of  three-phase  low  tension  circuits  for 
supplying  power  to  the  furnace. 

In  conclusion,  mention  must  be  made  of  the  Booth-Hall 
furnace,  which  combines  the  essential  features  of  the  Eennerfelt 
and  original  Stobie  furnaces.  In  this  historical  review  it  has 
only  been  possible  to  mention  those  types  of  furnaces  which  are 
either  in  use  at  the  present  day,  or  which  have  in  the  past  de- 
monstrated certain  principles  that  are  now  the  foundation  of 
more  widely  known  and  later  types. 

1  British  Specification,  No.  106,626,  March,  1916. 

2  Ibid.,  No.  118,233,  1918;  No.  121,563,  1918. 


CHAPTEE  II. 

ELECTEICAL  DEFINITIONS. 

I.  Alternating1    Current    Supply. — A    source    of    electrical 
energy   is    "  alternating "    when   the    voltage    passes    through 
periodic   variations   of   magnitude,   together   with   reversal   of 
sign. 

II.  Alternating  Current. — If  a  source  of  alternating  electrical 
energy  is  connected  to  a  suitable  circuit,  a  current  will  flow 
the  magnitude  and  direction  of  which  will,  at  all  moments,  vary 
directly  as  the  voltage. 

III.  Wave  Form  or  Wave  Shape. — The  wave  form  of  an 
alternating  current  is  found  by  plotting  a  curve  with  instan- 


FIG.  31. 

taneous  values  of  current  as  ordinates  against  corresponding  time 
values  as  abscissae.  The  wave  form  of  the  impressed  voltage 
may  similarly  be  plotted.  Owing  to  the  characteristic  design  of 
alternating  current  generators,  the  wave  form  approximates  to 
that  of  a  sine  wave,  which  can  be  developed  by  simple  geo- 
metrical means. 

Kef  erring  to  Fig.  31,  imagine  a  point  P  revolving  on  the 
circumference  of  a  circle  AYBXA  at  a  uniform  speed  and  in  a 
clock-wise  direction  from  X  through  AYB,  and  back  again  to 
X;  then,  by  plotting  the  vertical  distance  of  the  point  P  from 
the  horizontal  diameter  XY  against  time,  or  corresponding 

(42) 


ELECTRICAL  DEFINITIONS 


43 


angular  displacement  from  a  time  axis  XY,  a  curve  XAYBX 
will  be  obtained. 

IV.  Cycle. — Kef  erring  again  to  Fig.  31,  the  revolving  point 
P,  starting  from  any  position  with  a  uniform  velocity,  will  have 
performed  one  complete  cycle  when  it  has  returned  to  its  start- 
ing point,  having  made  one  complete  circuit. 

One  cycle  is  therefore  completed  in  360°  of  angular  displace- 
ment. Used  in  a  purely  electrical  sense,  a  cycle  is  regular  and 
periodic.  A  "  cycle  "  is  represented  by  the  symbol  <—». 

V.  Electrical  Degree.— One  360th  part  of  a  cycle. 

VI.  Period. — The  time  required  to  complete  one  cycle. 

VII.  Frequency  or  Periodicity. — The  number  of  cycles  per- 
formed in  one  second. 


FIG.  33. 

In  electrical  equations  frequency  is  denoted  also  by  the 
symbol  «-^. 

VIII.  Phase. — If  a  point  P  revolves  about  a  centre  0  (Fig. 
32)  then  the  phase  of  that  point,  relative  to  any  time  axis  AO,  is 
its  angular  displacement  from  that  axis. 

Again,  if  two  points  P  and  P'  revolve  about  the  same  centre 
O  with  the  same  frequency  but  not  necessarily  equal  radii,  and 
if  the  angle  between  OP  and  OP'  equals  <£°,  then  the  points  P 
and  P'  are  said  to  be  <f>°  "  out  of  phase  ". 

By  plotting  two  wave  curves  for  these  points  P  and  P' 
(Fig.  33),  representing  a  complete  cycle  and  beginning  at  simul- 
taneous .moments  when  these  points  are  at  given  vertical 
distances  from  XY  and  <f>°  out  of  phase,  it  will  be  seen  that  the 
wave  curve  for  point  P  is  always  <j>°  out  of  phase  with  the  curve 
for  P'.  In  this  particular  instance  the  plotting  of  the  wave 
curves  has  been  commenced  at  a  moment  when  the  point  P  is 


44  THE   ELECTEO-METALLUEGY   OF    STEEL 

0°  and  the  point  P'(0°  +  </>°)  past  their  maximum  positive  value 
at  A. 

These  wave  curves  might  equally  well  have  been  obtained 
by  plotting  instantaneous  values  of  voltage  against  time  for  two 
separate  sources  of  alternating  current  of  similar  frequency,  but 
</>°  out  of  phase  with  one  another  and  having  voltages  of  differ- 
ent maximum  values. 

It  will  be  shown  later  how  two  or  more  alternating  currents 
of  the  same  frequency,  but  out  of  phase,  may  be  combined  to 
produce  a  resultant  effect  in  the  same  way  that  forces  of  equal 
or  unequal  magnitude,  acting  on  a  body  at  different  angles,  may 
be  compounded  by  graphical  methods  to  produce  one  resultant 
force. 

IX.  Root = Mean -Square  or  Effective  Values  of  Voltage  and 
Current. — It  has  been  stated  (see  Def.  II.)  that,  when  a  suitable 
closed  circuit  is  connected  to  an  alternating  current  supply,  a 
current  will  flow  whose  magnitude  and  direction  at  any  moment 
will  depend  upon  the  instantaneous  value  of  the  voltage  as 
regards  both  magnitude  and  sign. 

Since  the  voltage  across  the  terminals  of  such  a  closed  cir- 
cuit undergoes  periodic  variation  of  magnitude  and  reversal  of 
sign,  it  may  be  represented  by  a  sine  wave  curve ;  likewise  the 
current  flowing  through  the  circuit  may  be  similarly  represented. 

Now  the  physical  effects  of  any  electric  current  are  generally 
due  to  the  power  developed,  which  is — as  will  be  seen  later — 
proportional  to  the  -product  of  current  and  impressed  voltage. 
It  is  necessary,  therefore,  for  purposes  of  measurement  and  cal- 
culation, to  know  the  value  of  the  average  current  or  voltage 
which  would,  in  any  half  cycle,  produce  exactly  the  same  effect 
as  the  sum  of  all  the  instantaneous  current  or  voltage  values, 
which  rise  from  zero  to  a  maximum  positive  or  negative  value 
and  then  fall  back  again  to  zero. 

Again,  since  power   is   proportional   to  C  x  E,  and    either 

C  or  E  can  be  expressed  in  terms  of  one  another  and  circuit 

•p 
resistance  by  the  equation  C  =  =~,  it  follows  that  the  physical 

XV 

effect  produced  is  proportional  to  the  square  of  the  current  and 
likewise  to  the  square  of  the  voltage. 


ELECTEICAL  DEFINITIONS  45 

i.e.  if  the  power  W  (watts)  =  C  x  E 

then  W  =  C  x  CK  (R  =  resistance  of  the  circuit) 

=  C2R 
and  again,  if  power  W       =  C  x  E 

then  W      =  1  x  E 

XX 

=  E2 
R' 

From  this  reasoning  it  follows  that  the  mean  effective  value 
of  an  alternating  current  or  voltage  is  proportional  to  the  square 
root  of  the  mean  of  the  instantaneous  values  squared,  and  not 
to  the  mean  of  the  instantaneous  values  themselves. 

For  any  alternating  current  supply  whose  voltage  may  be 
represented  by  a  simple  sine  wave,  the  effective  values  of  voltage 
or  current  flowing  through  a  connected  circuit  are  equal  to  the 
maximum  or  crest  values  -r-  ^2.  Ammeters  and  voltmeters 
always  indicate  the  effective  values. 

X.  Lead  and  Lag. — Suppose  Fig.  33  represents  the  voltage 
or   current   curves  for  one  complete  cycle  of   two  alternating 
currents  of    similar   frequency,    but  differing   in   phase.     The 
maximum  values  of  the  curve  PT'  occur  exactly  cf>°  before  those 
of  curve  PP  and  are  said  to  "  lead  "  by  <£°,  whereas  correspond- 
ing values  of  PP  "  lag  "  0°  behind  PT'. 

If  the  curves  PT'  and  PP  represent  the  voltage  and  current 
waves  respectively  of  an  alternating  current,  then  the  current  is 
"  la*?gmg  "  $°  behind  the  voltage,  and  the  "  angle  of  lag"  is  <f>° ; 
similarly  the  current  may  sometimes  "lead"  the  voltage  and 
then  have  a  certain  "  angle  of  lead". 

XI.  Induction  and  Induced  Currents. — Whenever  a  current 
flows  through  a  conductor  a  magnetic  field  is  produced,  which, 
according  to  the  usual  convention,  is  said  to  contain  a  certain 
number  of  magnetic  lines  of  force,  whose  number  and  direction 
will  depend  upon  the  magnitude  and  direction  of  the  current 
and  the  permeability  of  the  medium  through  which  the  magnetic 
lines  of  force  pass.     The  conductor,  as  shown  in  Fig.  34,  may 
be  either  straight  or  coiled,  and  will  produce  magnetic  fields  in 
the  directions  indicated  when  carrying  a  current  likewise  shown. 

Conversely,  if  a  conductor  lies  in  a  magnetic  field  of  varying 
intensity  and  in  a  position  other  than  in  the  exact  direction  of 


46 


THE   ELECTEO-METALLUEGY   OF    STEEL 


the  lines  of  force,  then  a  voltage  will  be  induced  within  it,  the 
magnitude  and  sign  corresponding  at  any  moment  to  the  rate 
of  change  in  the  number  and  direction  of  the  magnetic  lines  of 
force  cutting  the  conductor;  no  voltage  is  induced  so  long  as 
the  magnetic  field  remains  unchanged. 

Such  a  phenomenon  is  known  as  "  induction,"  and,  accord- 
ing to  convention,  1  volt  will  be  induced  when  a  conductor 
cuts  magnetic  lines  of  force  at  the  rate  of  10s  lines  per 
second. 

Supposing  now  that  a  magnetic  field  of  varying  intensity 
and  direction  is  set  up  in  an  iron  ring  B  by  a  coiled  conductor 
A  carrying  an  alternating  current  (Fig.  35),  then  a  voltage 
will  be  induced  in  the  coiled  conductor  C,  which,  if  made  into 
a  closed  circuit,  will  carry  an  "induced  current"  alternating 


FIG.  34. 


FIG.  35. 


at  the  same  frequency  as  the  current  in  the  coil  A.  The  con- 
ductor that  sets  up  the  magnetic  field  is  generally  known  as  the 
"  primary  winding,"  and  that  in  which  the  voltage  is  induced 
as  the  "  secondary  winding  ".  The  induced  or  secondary  volts 
will  bear  the  same  ratio  to  the  exciting  or  primary  volts  as  the 
number  of  turns  of  the  respective  windings.  Each  turn  may 
be  regarded  as  a  separate  conductor,  so  that  increasing  the 
number  of  turns  of  the  secondary  is  equivalent  to  adding  further 
induced  voltage  in  series,  and  consequently  raising  the  total 
voltage  for  the  same  magnetic  field. 

If  a  certain  amount  of  power  is  utilised  in  a  primary  winding, 
it  is  obvious  that  only  the  same  amount  of  power  can  be  de- 
veloped by  the  secondary,  even  assuming  100  per  cent,  efficiency 
of  conversion.  Therefore,  since  the  primary  and  secondary 


ELECTEICAL   DEFINITIONS  47 

voltages  are  proportional  to  the  respective  number  of  turns,  the 
primary  and  secondary  currents  will  be  inversely  proportional, 
so  that  the  product  of  volts  and  amperes  or  volt-amperes  is 
equal  for  both  circuits.  This  constitutes  the  underlying  prin- 
ciple of  step-down  transformers,  to  which  type  all  induction 
furnaces  belong. 

XII.  Self-induction,  Reactance,  or  Reactive  Resistance.— 
When  an  alternating  current  flows  through  a  conductor,  either 
straight  or  in  the  form  of  a  coil  or  solenoid,  it  will  produce  a 
magnetic  field  constantly  changing,  as  regards  both  the  number 
of  lines  of  force  and  reversal  of  their  direction.  As  explained 
in  Def.  XL,  such  a  conductor  would  be  under  the  influence  of 
the  changing  magnetic  field,  and  an  alternating  voltage  would 
therefore  be  induced  within  it. 

The  simple  wave  form  of  an  alternating  current  shows  that 
the  maximum  rate  of  change  of  current,  and,  consequently,  of 
the  number  of  magnetic  lines  of  force  produced,  takes  place  at 
the  steepest  point  of  the  curve,  which  is  just  at  the  moment  of 
reversal.  Although  there  are  no  magnetic  lines  of  force  at  the 
exact  moment  of  reversal,  the  rate  of  diminution  or  increase  in 
number  is  then  at  a  maximum,  and  at  this  point,  therefore,  the 
"  self-induced  "  or  "  reactive  "  voltage  reaches  its  highest  value. 
It  follows,  therefore,  that  the  wave  forms  of  the  current  flowing 
and  the  self-induced  voltage,  if  plotted  simultaneously,  would 
be  90°  out  of  phase  and  have  the  same  frequency. 

The  result  of  such  self-induced  voltage,  which  is  out  of  phase 
with  the  applied  voltage,  is  to  reduce  the  power  of  the  latter  to 
force  current  through  the  conductor,  and  at  the  same  time  to 
cause  the  wave  curve  of  the  current  flowing  to  lag  behind  that 
of  the  applied  voltage.  This  latter  effect  is  important  in  con- 
nection with  "  wattless  current  "  and  power  factor.  The  self- 
induction,  reactance,  or  reactive  resistance  of  a  conductor  or 
circuit  is,  then,  a  property  by  virtue  of  which  it  offers  resistance 
to  the  flow  of  alternating  current.  Such  resistance  is  in  no 
way  due  to  the  material  of  which  the  conductor  is  composed, 
but  to  characteristics  which  promote  the  generation  of  a  self- 
induced  or  reactive  voltage  as  above  described. 

The  self-induction  or  reactance  of  any  given  circuit  is  equal 
to  2-7T  »^>  L  ohms.  The  co-efficient  of  self-induction  or  inductance 


48  THE   ELECTRO-METALLURGY   OF   STEEL 

(symbol  L  equals  one  Henry)  is  present  when  one  ampere 
produces  108  interlinkings  of  lines  of  force  and  windings  (i.e. 
number  of  turns  x  lines  of  force  per  second). 

The  number  of  lines  of  force  or  the  magnetic  flux  produced 
by  a  current  of  one  ampere  may  be  calculated  from  various 
formulae  according  to  the  nature  of  the  electric  and  magnetic 
circuits. 

XIII.  Reactance  Voltage  or  E.M.F.  of  Self  =  Induction. —The 

self-induced  E.M.F.  or  voltage  may  be  calculated  as  the  product 
of  current  and  reactance,  and  is  equal  to  2?r  —>  LC. 

XIV.  Percentage  Reactance  Drop. — The  term  "  percentage 
reactance  drop"  is  used  for  expressing  the  reactance  voltage  in 
a  circuit  as   a   percentage  ratio  of  the  voltage  applied  to  the 
circuit,  usually  at  normal  full  load  current. 

XV.  Wattless  Current. — When  the   voltage    and    current 
curves  of  an  alternating  current  are  in  phase,  the  entire  "  effec- 
tive "  or  root-mean-square  current  value  is  useful  for  doing  work 
as  electrical  energy.     This  is  not  true  for  a  current  which  "lags  " 
or  "  leads"  the  voltage,  when  only  that  component  of  the  cur- 
rent which  is  in  phase  with  the  voltage  is  capable  of  doing 
useful  work,  the  other  component  being  "wattless  ". 

It  should  be  here  mentioned  that  a  wave  curve  of  current 
or  voltage  may  easily  be  resolved  graphically  into  two  compon- 
ents, and  in  the  same  way  two  waves  of  similar  frequency,  but 
differing  in  phase  or  magnitude,  may  be  compounded  to  produce 
one  wave  curve. 

XVI.  Volt = ampere. — The  product  of  the  effective  or  measured 
value  of  volts  across  a  circuit  and  the  effective  current  value  in 
the  circuit  is  expressed  in  terms  of  volt-amperes  (V.A.)  or,  when 
divided  by  1000,  of  kilo-volt-amperes  (K.V.A.). 

XVII.  Power  Factor. — The  power  factor  of  a  circuit  in  which 
electrical  energy  is  transformed  or  dissipated,  is  the  cosine  of 
that  angle — usually  denoted  by  $° — by  which  the  current  "  lags  " 
or  "  leads  "  the  impressed  voltage.     Power  factor  is  of  great 
importance  in  all  electric  furnace  installations,  and,  except  in 
certain  cases  where  considerable  circuit  reactance  is  purposely 
introduced,  should  approach  unity  as  far  as  possible  at  normal 
loads. 


ELECTEICAL   DEFINITIONS  49 

The  power  factor  of  a  circuit  indicates  the  ratio  of  useful 
current  flowing  to  the  total  current,  and  since  the  capacity  of 
transformers  and  generators  is  limited  by  the  permissible  cur- 
rent, it  is  obvious  that  the  power  factor  represents  the  ratio  of 
actual  power  output  to  the  maximum  possible  output.  In  other 
words,  if  the  power  factor  is  *6,  then  only  '6  of  the  power  plant 
capacity  is  available  for  doing  useful  work. 

XVIII.  Watt  and  Kilowatt. — The  true  power  absorbed  in  a 
circuit  is  measured  in  terms  of  watts  or  kilowatts,  and  is  the 
product  of  volt-amperes  or  kilo-volt-amperes  and  power  factor, 

i.e.  Watts  =  V  x  A  x  cos  <£°  (where  <£°  is  the   angle   of 

"lag"  or  "lead"), 

Kilowatts  =  V  x  A  x  cos  <£°  -=-  1000. 

If  the  current  and  voltage  wave  forms  are  in  phase,  then 
cos  0°  =  1,  and  the  volt-amperes  are  equivalent  to  watts. 

XIX.  Surging. — Strictly   speaking,    the   term    "surging" 
should  be  used  only  to  express  a  more  or  less  constant  and 
periodic  current  or  voltage  fluctuation,  but  for  furnace  loads  may 
be  more  loosely  used  to  denote  persistent  current  instability  or 
fluctuation  of  considerable  magnitude.    In  the  case  of  direct  arc 
furnaces  it  sometimes  happens  that  the  nature  of  the  charge  is 
such  that  a  steady  load  is  most  difficult  to  maintain.     This  con- 
dition is  most  likely  to  occur  with  a  loosely  packed  charge  of 
heavy  and  irregular  shaped  scrap,  and  results  from  the  con- 
stantly varying  resistance  of  the  charge.     This  is  due  to  local 
fusion  of  the  metal  by  the  arcs  formed  at  the  various  points  of 
contact,  which  may  either  facilitate  or  interrupt  the  passage  of 
current.     When  the  resistance  of  a  charge  of  scrap  is  very  low, 
slight  movement  of  the  electrodes  may  be  sufficient  to  cause 
sudden  rushes  of  current,  which  may  at  times  reach  values  100 
per  cent,  in  excess  of  the  normal  full  load  current.     Surging  is 
sometimes  experienced  even  when  the  entire  charge  is  molten, 
and  is  then  probably  due  to  a  phenomenon  known  as  "  pinch 
effect,"  which  occurs  in  the  slag  covering  ;  this,  however,  only 
happens  when  the  electrode  is  almost,  if  not  actually,  in  contact 
with  certain  basic  and  acid  slags. 

Surging  is  highly  objectionable  for  many  reasons,  some 
simple  and  some  very  complex.  So  long  as  a  current  is  violently 


50  THE    ELECTRO-METALLURGY   OF    STEEL 

fluctuating  it  is  only  possible  to  prevent  it  reaching  an  in- 
stantaneous value,  large  enough  to  automatically  open  the  main 
switch,  by  operating  the  furnace  at  a  considerably  reduced  aver- 
age load.  The  installation  would  not  then  be  working  at  its 
full  capacity,  and  reduction  of  output  accompanied  by  an  in- 
crease in  power  and  other  costs  would  result.  Surging  or  cur- 
rent fluctuation  can  to  a  great  extent  be  reduced,  if  not  prevented, 
by  increasing  the  self-induction  of  the  load  circuit,  which  can 
be  done  by  the  aid  of  reactance  or  choking  coils. 

XX.  Single=phase   Alternating  Current. — The   alternating 
current  flowing  through  a  single  circuit  by  virtue  of  an  alternat- 
ing voltage  is  known  as  a  single-phase  current.     A  source  of 
single-phase  current  can  only  have  one  wave  curve  for  its  im- 
pressed voltage.     (See  also  Def.  I.) 

XXI.  Two-phase  Alternating  Current. — A  so-called  "  two- 
phase  current"  denotes  the  availability  or  application  of  two 
sources  of  single-phase  current  of  similar  frequency  and  voltage 
magnitude,  but  generally  differing  in  phase  by  90°.     This  par- 
ticular phase  displacement  permits  of  a  useful  combination  of 
two  such  single-phase  currents  flowing  through  separate  circuits 
for  different  purposes.     The  two  wave  curves  of  the  individual' 
impressed  voltages  are  represented  in  Fig.  36.     Such  alternating 
currents  are  also  said  to  be  in  "  Quadrature,"  because  the  phase 
displacement  is  one-quarter  of  a  cycle. 


90* 


FIG.  36. 

XXII.  Three=phase  Alternating  Current.  —  A  so-called 
"three-phase  current"  denotes  the  availability  or  application 
of  three  sources  of  single-phase  current  of  similar  frequency  and 
voltage  magnitude  but  differing  in  phase  by  120°.  Again,  this 
phase  displacement  permits  of  a  variety  of  combinations  of  three 
such  single-phase  currents  flowing  through  separate  circuits, 
which  may  be  for  lighting,  heating,  power  generation  or  con- 


ELECTKICAL   DEFINITIONS 


51 


version  to  two-phase  and  even  four-phasfe  current.  The  wave 
curves  of  the  individual  impressed  voltages  are  shown  by 
Fig.  37. 


FIG.  37. 

XXIII.  Load  Factor. — This  term  must  not  be  confused  with 
"power  factor".  The  percentage  ratio  of  average  power  de- 
mand to  the  maximum  power  demand,  during  any  given  period 
of  time,  is  the  "  load  factor  "  of  the  installation  during  that 
period.  If  a  furnace  installation,  which  has  a  maximum  power 
absorbing  capacity  of  1000  kw.,  is  actually  supplied  with  an 
average  of  only  800  kw.  during  a  certain  period,  then  the  load 
factor  is  80  per  cent,  over  that  period.  The  load  factor  is  of 
the  very  greatest  importance  in  furnace  work,  as  it  is  only  by 
working  as  nearly  as  possible  at  the  full  plant  capacity  that  the 
maximum  economy  of  production  can  be  attained. 

The  load  factor  of  an  installation  is  sometimes  averaged 
over  a  period  of  a  month  or  year,  which  will  include  all  delays 
and  stoppages.  In  any  case,  when  referring  to  load  factor,  the 
period  upon  which  it  is  calculated  should  be  clearly  stated, 
when  not  otherwise  understood. 


CHAPTEE  III. 

APPLICATION  OF  SINGLE  AND  POLYPHASE  CUKRENTS 
TO  FURNACE  OPERATION. 

Application  of  Single-phase  Current. — It  has  been  shown 
how  a  single-phase  current  may  be  produced  in  a  circuit  by  a 
single  impressed  voltage  that  is  passing  through  periodic  changes 
of  magnitude  and  sign.  An  outgoing  current  must  have  a  re- 
turn to  complete  its  circuit,  otherwise  no  current  flows,  so  that 
at  least  two  main  leads  are  required  to  supply  energy  to  a 
furnace  from  any  source  of  single-phase  current.  The  nature 
of  the  circuit  and  the  manner  in  which  the  power  is  absorbed 
depends  upon  the  particular  furnace  design.  The  various 


FIG.  38. 


FIG.  39. 


methods  of  using  single-phase  current  for  arc  furnaces  from  a 
two-wire  supply  are  shown  in  Figs,  38,  39,  and  40,  which  also 
illustrate  the  principles  of  direct  and  indirect  arc  heating  used 
in  certain  modern  furnaces. 

Fig.  41  illustrates  in  plan  the  heat  developing  and  power 
supply  circuits  of  a  single-phase  induction  furnace,  A  being  the 
laminated  transformer  core  in  section,  B  the  primary  windings 
from  the  main  supply  leads,  and  C  the  short-circuited  secondary 
coil  in  which  the  low  tension  current  is  induced. 

A  three-wire  system  was  used  in  the  Giffre  furnace,  the 

(52) 


SINGLE  AND  POLYPHASE   CURRENTS  TO   FUENACE  OPERATION     53 

third  wire  being  connected  from  the  middle  terminals  of  two 
single-phase  alternators  to  a  bottom  electrode,  as  shown  in 
Fig.  42.  The  single-phase  alternators,  mechanically  coupled 
together,  are  connected  in  series  and  generate  synchronously  at 
the  same  frequency  and  voltage.  If  the  terminal  voltage  of 
each  alternator  is  50  volts,  then  the  arc  voltage  will  be  50, 


FIG.  40.  FIG.  41. 

assuming  the  two  arcs  are  balanced.  If  the  electrodes  are  at 
any  time  unbalanced,  then  a  current  will  flow  through  the 
bottom  electrode  back  to  the  middle  terminal. 

Application  of.  Two«phase  Currents. — It  is  more  economical 
to  generate  and  transmit  two-phase  current,  owing  to : 

(i)  The  reduced  cost  of  the  generating  plant. 

(ii)  The  saving  of  copper  in  conductors. 


FIG.  42. 

It  has  been  explained  that  a  two-phase  system  is  simply  a 
suitable  combination  of  two  single-phase  currents,  90°  out  of 
phase,  which  can  be  readily  compounded  to  produce  a  resultant 
current  in  a  common  return  conductor.  A  two-phase  supply, 
then,  might  constitute  two  entirely  separate  single-phase  circuits 
with  impressed  voltages  of  similar  frequency  and  magnitude, 
but  90°  out  of  phase.  If  the  separate  currents  were  utilised 


54 


THE   ELECTRO-METALLUKGY   OF    STEEL 


in  a  machine  or  furnace  and  kept  distinct  throughout,  it  is 
obvious  that  four  wires  would  be  required  to  transmit  the  two 
currents.  Two  such  circuits  A  and  B,  in  which  single-phase 
currents  are  being  generated  90°  out  of  phase,  are  represented  in 
Fig.  43.  If,  for  example,  the  voltage  across  A  is  at  its  maximum 
positive  value  of  100  volts  at  terminal  D  and  just  beginning  to 
fall  off,  then  the  voltage  across  the  terminals  of  B,  if  lagging  by 
90°,  will  be  zero  and  just  reversing  to  a  positive  sign  at  one 
terminal,  say  D'.  In  practice  it  is  customary  to  connect  one 
terminal  of  each  circuit  together,  and  to  connect  the  remaining 
"  outer"  terminals  to  two  corresponding  terminals  of  the  power 
absorbing  circuits.  The  latter  are  likewise  connected  together 
at  a  common  terminal,  which  is  connected  to  the  corresponding 


V   V    V    V    V  V 

D 

'  VV  V  VV  V 

C* 

D' 

1       +IOOV  0              0 

FIG.   43. 

FIG.  44. 


terminal  of  the  generating  circuits  by  a  common  return  con- 
ductor (Fig.  46).  In  this  way  only  three  conductors  are  used 
in  place  of  four,  which  constitutes  a  saving  of  copper,,  although 
the  "neutral"  return  conductor  carries  a  heavier  current  than 
that  flowing  through  the  individual  circuits.  In  Fig.  44  the 
heavy  dotted  line  EFGH  shows  the  resultant  current  wave  of 
the  two  current  waves  CD  and  C'D'  for  the  circuits  A^nd  B  in 
Fig.  43.  The  effective  value  of  this  curve  equals  N/12  its  crest 

value,  and  is  ~-  times  the  sum  of  the  effective  current  values 

of  the  wave  curves  CD  and  C'D'. 

Keferring  again  to  Fig.  43,  if  the  terminals  C  and  D',  or 
C  and  C'  were  connected  together  to  form  the  terminal  of  the 
return  conductor,  the  resultant  effect  for  furnace  work  would  be 
the  same.  It  should  be  carefully  borne  in  mind  that  such  inter- 


SINGLE    AND   POLYPHASE   CUEEENTS  TO  FUENACE  OPEEATION     55 


changeability  of   terminal   connections  does  not  apply  in  the 
case  of  three-phase  circuit  combinations. 

It  is  equally  possible  to  compound  two  single-phase  circuits 
which  differ  in  phase  by  60°  in  place  of  90°.  Two  such  single- 
phase  circuits  may  be  obtained,  for  example,  by  taking  current 
from  two  separate  phases  of  a  three-phase  system,  as  was  sug- 
gested by  Girod  in  1910. 

The  various  systems  of  two-phase  connections  adopted  for 
furnace  operation  will  now  be  mentioned  in  the  order  in  which 
they  have  been  successively  introduced. 

Heroult  Two- phase  System. — It  was  proposed  in  1905  to 
use  two-phase  current  for  supplying  power  to  the  electric  mixing 
furnace  patented  in  that  year,  in  which  case  the  circuits  would 
be  arranged  as  shown  by  Fig.  45.  As  a 
three-phase  system  presented  considerable 
advantages  over  the.  two-phase,  the  latter 
was  never  applied  in  practice,  but  is  in- 
teresting in  view  of  recent  furnace  develop- 
ment. 

Electro -metals  Two -phase  Three -wire 
System. — The  two-phase  low  tension  cir- 
cuits are  represented  in  Fig.  46  by  the 
windings  A  and  B  drawn  at  right  angles, 
according  to  convention,  showing  them  to 
be  90°  out  of  phase.  The  two  phases  are 
connected  at  the  common  neutral  point  0,  from  which  a  neutral 
conductor  goes  to  an  electrode  in  the  furnace  bottom.  Each 
upper  electrode  is  connected  to  the  outer  terminal  of  one  phase- 
The  hearth  only  becomes  a  conductor  when  hot,  and  will  then 
carry  practically  the  full  return  current  equivalent  to  ,/2  of  the 
current  flowing  through  each  electrode  when  balanced.  When 
cold  the  bottom  is  non-conductive  and  the  two  electrodes  then 
operate  in  series.  The  two  phases  A  and  B,  which  have  simi- 
larly only  a  series  connection,  combine  to  produce  a  resultant 
effect  equivalent  to  a  simple  single-phase  power  generating 
circuit,  the  voltage  across  the  electrodes  being  1'41  times  the 
voltage  across  either  phase  winding  A  or  B. 

The  two-phase  low  tension  supply  may  be  generated  for  each 
furnace  by  generators,  but  is  usually  transformed  down  by  static 


FIG.  45. 


56 


THE    ELECTROMETALLURGY    OF    STEEL 


transformers  to  'the  required  voltage  from  either  a  two  or  three- 
phase  high  tension  system;  in  the  latter  case  the  transformer 
windings  are  connected  in  the  well-known  Scott  method,  which 
gives  perfect  balance  on  the  three  supply  phases,  provided  the 
secondary  side  is  properly  loaded.  Figs.  46  and  47  show  the 
loading  on  the  primary  side  of  a  three-phase  system  after  and 
before  the  hearth  becomes  fully  conductive  (Hill  and  Fleming).1 
Qirod  Two-phase  Connection  (1910). — Instead  of  using 
two-phase  current  90°  out  of  phase,  Girod  proposed  to  transform 
from  two  phases  only  of  a  three-phase  system.  Normally  the 


FIG.  46. 


FIG.   47. 


FIG.  48. 

two  low  tension  phases  will  be  60°  apart  if  connected  together 
symmetrically,  but  by  reversing  one  phase,  the  phase  displace- 
ment will  then  be  120°.  Consequently,  the  low  tension  two-phase 
currents  will  be  120°  out  of  phase  in  place  of  the  usual  90°  for 
two-phase  systems.  By  this  arrangement  (Fig.  48),  if  the 
voltage  between  each  top  and  bottom  electrode  is  55  volts,  then 
the  voltage  between  the  upper  electrodes  will  be  95  volts,  and  a 
current  will  flow  not  only  from  top  to  bottom,  but  between  the 
upper  electrodes  also.  This  results  in  the  load  being  nearly 
equally  balanced  in  the  three  supply  phases,  when  the  load  is 
equally  divided  between  the  two  arc  circuits. 

1  Trans.  Faraday  Soc.,  Jan.,  1919. 


SINGLE  AND   POLYPHASE   CURRENTS  TO    FURNACE  OPERATION     57 


The  furnace  low  tension  connections,  it  will  be  seen,  closely 
resemble  both  the  Electro-metal  and  Stobie  systems. 

Stobie  Two -phase  Furnace  Connections. — In  this  system 
the  common  return  wire  of  the  Electro-metals  furnace  is  re- 
placed by  separate  leads  from  the  two  phases  to  two  bottom 
electrodes,  as  shown  in  Fig.  49.  A  and  B  represent  the  two- 
phase  circuits,  whose  outer  terminals  are  connected  to  electrodes 
C  and  D  ;  the  return  wires  of  the  phases  A  and  B  are  separately 
connected  to  bottom  electrodes  E  and  F  respectively,  which  are 
embedded  in  a  lining  that  is  conductive  only  when  hot. 

This  arrangement  is  specially  designed  to  lengthen  the  path 
of  current  through  the  bath  for  the  purpose  of  improving  the 
metallurgical  operations  by  better  distribution  of  heat  and 
circulation  of  the  metal.  The  return  wires  are  not  connected 

outside  the  furnace,  and  there 

is  no  neutral  point  common  to 
the  two  phases  until  the  lining 
becomes  conductive.  Under 
these  conditions  the  furnace 
cannot  be  operated  without 
pre-heating  by  gas  or  other 
means  to  render  the  bottom 
conductive.  As  soon  as  the 
hearth  becomes  a  conductor,  the  two  bottom  electrodes  are 
electrically  connected,  and  the  conditions  are  then  somewhat 
similar  to  those  obtaining  in  the  Electro-metals  furnace. 
The  path  of  current,  however,  from  the  top  to  the  bottom 
electrodes  must  be  to  some  extent  different,  but  the  actual 
direction  is  problematical,  since  the  bottom  electrodes  are  elec- 
trically connected  through  both  the  conductive  hearth  and  the 
bath  of  metal,  which  together  constitute  a  neutral  point  of  the 
two  circuits. 

Rennerfelt  Two-phase  Furnace  Connections. — Two-phase 
current  is  here  employed  to  form  two  arcs  which  are  entirely 
independent  of  the  furnace  charge  or  lining  for  any  part  of 
their  circuit.  The  combination  of  phases  is  similar  to  that  used 
in  the  electro-metals  furnace,  but  the  neutral  conductor,  instead 
of  being  connected  to  a  conductive  lining,  is  attached  to  a  third, 
vertically  suspended  electrode  on  to  the  end  of  which  the  arcs 


FIG.  49. 


58 


THE    ELECTBO-METALLUKGY   OF    STEEL 


strike  from  two  other  electrodes.  The  arrangement  is  shown 
diagrammatically  by  Fig.  50,  the  two-phase  low  tension  current 
being  transformed  from  three-phase  by  Scott-connected  trans- 
formers. 

In  certain  cases,  the  neutral  electrode  C  may  rest  on  the 
furnace  charge  or  actually  dip  into  the  bath,  when  the  condi- 
tions would  be  such  that  two  separate  direct  arcs  are  formed. 

Dixon  Two-phase  Furnace. — Heroult  had  originally  pro- 
posed to  split  a  two-phase  current  supply  into  two  entirely 
separate  circuits,  each  having  two  upper  electrodes  arranged  in 
series,  and  thus  eliminate  all  bottom  electrode  connections. 
Such  a  furnace  would  resemble  two  single-phase  Heroult  fur- 


FIG.  50. 


FIG.  51. 


naces  assembled  in  one  body.  A  single-phase  circuit  with  two 
arcs  in  series,  supplied  with  power  from  a  source  at  constant 
voltage,  requires  more  complicated  regulation  than  if  connected 
to  an  alternator  designed  with  a  "  drooping  characteristic," 
since  not  only  must  the  voltage  be  equally  divided  between  each 
pair  of  arcs  in  series,  but  the  current  flowing  through  the 
circuit  must  likewise  be  controlled. 

The  system  of  connections  adopted  by  Dixon  (Fig.  51)  was 
designed  to  simplify  such  complicated  load  regulation  of  each 
electrode.  The  two  phases  A  and  B  are  connected  together  at 
their  middle  points  by  a  conductor,  the  outer  terminal  ends  of 
the  two  phases  being  connected  to  electrodes  C,  D,  E,  and  F. 
It  will  be  seen  that  the  current  leaving  any  electrode  will  have 


SINGLE  AND  POLYPHASE   CURRENTS  TO  FURNACE  OPERATION     59 

more  than  one  return  path,  so  that  any  variation  of  the  current 
flowing  through  one  will  be  distributed  between  at  least  two 
others.  By  this  arrangement  the  regulation  of  each  electrode 
can  be  effected  solely  by  the  current  flowing  in  its  circuit. 

Several  methods  of  compounding  two-phase  currents  sup- 
plied to  furnaces  of  the  conducting  hearth  type  were  introduced 
at  a  prior  date,  and,  for  a  detailed  description  of  the  numerous 
ways  adopted,  reference  should  be  made  to  British  Specification 
No.  4742,  1914. 

Booth- Hall  Furnace  Connections. — The  electrical  features 
of  this  furnace  (Fig.  122) l  embody  the  combined  principles  of  the 
Eennerfelt  and  Stobie  furnaces.  The  impossibility  of  starting 
a  Stobie  two-phase  conductive  hearth  furnace  when  cold  has 
been  mentioned,  and  to  overcome  this  objection  a  third  electrode 
is  introduced,  which  serves  as  a  neutral  return  for  the  current 


+U2 


B 


A 


FIG.  52. — Three-phase  wave  curves. 

flowing  through  the  phase  electrodes.  When  starting  to  melt, 
this  auxiliary  electrode  rests  upon  the  charge,  and  is  later  re- 
moved as  soon  as  the  hearth  is  sufficiently  conductive  to  carry 
the  full  return  current. 

Application  of  Three-phase  Currents. — It  has  been  shown 
how  two-phase  currents  may  be  compounded  to  produce  a  re- 
sultant flow  in  a  neutral  return  wire  of  a  three-wire  system. 
The  same  case  applies  to  three-phase  currents,  which,  however, 
may  be  compounded  in  several  different  ways. 

Usually  three-phase  currents  are  primarily  generated  120° 
out  of  phase,  and  the  voltage  curves  may  be  represented  by 
Fig.  52.  Three  such  separate  single-phase  currents  may  have 
entirely  independent  circuits  throughout,  and  in  that  case  three 
distinct  pairs  of  wires  would  be  required  to  carry  the  separate 


Met.  and  Chem.  Eng.,"  Vol.  XXIII,  p.  212. 


60 


THE    ELECTRO-METALLURGY   OF    STEEL 


currents  from  the  point  of  generation  to  a  furnace,  or  other 
power-absorbing  apparatus,  and  back  again. 

The  curves  A,  B,  and  C  represent  three  voltage  wave  forms 
of  equal  magnitude  and  frequency,  but  120°  out  of  phase.  At 
any  given  moment  M  on  the  time  axis  XY,  there  will  be  definite 
values  of  impressed  voltage  for  each  circuit,  which  are  repre- 


sented in  Fig.  53  as  being  +  6,  +  6,  and  -  12  for  the  circuits  A, 
B,  and  C  in  reference  to  the  imaginary  scale  indicated  in  Fig. 
52.  The  voltage  across  circuit  A  is  falling,  that  across  B  rising, 
and  that  across  C  is  at  its  maximum  negative  value,  but  be- 
ginning to  fall  towards  zero  prior  to  reversing  in  sign  to  positive. 

The  algebraic  sum  of  three  such  im- 
pressed voltages  at  any  given  moment  is 
always  zero. 

Now  supposing  the  terminals  D,  F, 
and  H  are  connected  together,  then  the 
circuits  A,  B,  and  C  may  be  represented 
in  the  conventional  manner  as  shown  in 
Fig.  54,  demonstrating  them  to  be  120° 
out  of  phase.  The  point  whera  the 
three  terminals  D,  F,  and  H  join  is 
known  as  the  "  Star  point,"  and  the 
circuits  are  said  to  be  "  star  con- 
nected ". 

If  equal  resistances  r  are  put  in  circuit  between  the  outer 
terminals  E,  G,  and  I  and  a  common  conductor  Q  which  con- 
nects the  three  resistances  to  the  "  star  point  "  of  the  circuits 
A,  B,  and  C,  then  at  a  given  moment  a  current  will  flow  through 
each  circuit  whose  magnitude  and  direction  is  exactly  propor- 
tional to  the  magnitude  and  sign  respectively  of  the  impressed 
voltage.  If  the  voltages  at  the  terminals  E,  G,  and  I  are  4-  6, 
+  6,  and  --  12  respectively  at  a  given  moment,  then  currents 


FIG.  54. 


SINGLE  AND  POLYPHASE   CURRENTS  TO  FURNACE   OPERATION     61 

will  flow  through  the  circuits  ED,  GF,  and  IH  in  the  direction 
of  the  arrows,  and  with  magnitudes  of  6,  6,  and  12,  supposing 
the  resistances  r  are  each  equal  to  unity  ;  in  this  case  the 
current  through  HI  will  act  as  the  return  for  the  combined 
currents  DE  and  FG.  This  is  equally  true  at  any  other  moment 
when  the  resistances  r  are  equal,  and  the  result  is  that  no 
current  flows  through  the  neutral  conductor  Q.  This  means 
that  the  "  effective  "  or  measured  currents,  as  opposed  to  in- 
stantaneous values,  are  also  equal,  and  under  such  conditions 
the  load  is  said  to  be  "  balanced  ". 

Now  supposing  the  resistance  r  in  series  with  the  circuit 
C  is  increased,  then. the  current  flowing  in  that  circuit  will  be 
reduced,  and  will  be  less  than  the  sum  of  the  currents  through 
the  circuits  A  and  B  at  the  particular  moment  previously  con- 
sidered. The  neutral  conductor  Q  will  then  act  as  a  return  for 
part  of  the  current  flowing  through  A  and  B,  and  the  system 
will  then  be  "  out  of  balance,"  and  the  three  circuits  A,  B,  and 
C  will  not  be  carrying  the  same  "  effective  "  or  measured  cur- 
rent. 

The  main  line  wires  of  a  star-connected  power-generating 
system  are  connected  to  the  outer  terminal  of  each  phase,  and 
may  be  connected  to  a  three-phase  apparatus  absorbing  the 
electrical  energy  so  that  each  power-absorbing  circuit  of  that 
apparatus  is  in  series  with  one  line  wire,  the  three  outer  ends  of 
the  circuits  being  themselves  connected  to  a  common  star  point. 

It  will  be  seen  that  the  three  resistances  r  in  Fig'.  54 
are  so  connected  to  a  common  neutral,  and  are,  therefore,  star- 
connected. 

In  all  three-phase  direct  arc  furnaces  the  electrodes  or  line 
conductors  are  similarly  "star"  connected  through  the  arcs  to 
a  common  star  or  neutral  point,  which  is  the  metallic  charge  or 
bath  of  molten  metal.  In  furnace  work  the  charge  or  bath, 
which  so  constitutes  the  neutral  point,  is  sometimes  connected 
through  either  a  conducting  hearth  or  bottom  electrode  to  a 
neutral  return  conductor,  which  is  itself  connected  to  the  com- 
mon point  of  a  star-connected  generating  system.  As  has  been 
explained,  this  neutral  return  conductor  only  carries  current 
when  the  load  is  not  equally  distributed  or  balanced  between 
the  three  power-absorbing  circuits. 


62 


THE    ELECTKO-METALLTTRGY   OF    STEEL 


Star  Point 


If,  in  a  balanced  three-phase  star-connected  system,  the 
effective  or  measured  voltage  across  the  terminals  of  each  power- 
generating  phase — or,  in  other  words,  between  the  star  point 
and  the  outer  terminals  of  each  phase — is  equal  to  100  volts,  then 
the  "  line  voltage,"  or  the  voltage  between  any  pair  of  the  outer 
terminals,  will  be  ^3  times  100  volts  or  173  volts. 

If  the  three  power-absorbing  resistances  such  as  arcs  are 
star-connected,  as  is  the  case  in  all  direct  arc  furnaces,  the  voltage 
across  each  resistance  or  arc  will  be  the  "  line  voltage  "  -r-  \/3, 
when  the  load  is  equally  balanced.  Knowing  this  relationship 
between  line  voltage  and  arc  voltage,  it  is  easy  to  calculate  the 

total  load  on  a  three-phase  star- 
connected  direct  arc  furnace,  if  the 
current  flowing  through  each  of  the 
three  arcs  is  known  and  is  the  same 
for  all,  which  indicates  balance. 
Thus,  if  a  furnace  is  supplied  with 
power  from  transformers,  the  line 
voltage  will  be  invariable  for  given 
loads,  and  the  arc  voltage  will  be 
this  voltage  divided  by  1*73.  The 
total  load  in  K.V.A.  will  then  be 
the  current  flowing  through  each  arc 
multiplied  by  the  arc  voltage  x  3. 
This  rule  only  requires  knowledge 
of  the  line  voltage  and  the  current 
flowing  through  each  arc,  and  is  quite  independent  of  the  trans- 
former grouping.  Fig.  55  illustrates  the  nomenclature  used,  and 
shows  the  low  tension  generating  phases  star-connected. 

Delta  or  Mesh  Grouping*. — Referring  again  to  the  instan- 
taneous voltage  values  of  the  three  wave  forms  A,  B,  and  C 
(Fig.  52)  at  a  moment  M  on  the  time  axis  XY,  the  three  im- 
pressed voltages  may  be  graphically  represented  by  a  clock  dia- 
gram (Fig.  56),  which  shows  their  instantaneous  magnitude  and 
phase  displacement.  Since  the  amplitudes,  which  represent  the 
maximum  voltages  of  the  three  wave  forms,  are  equal,  the  radii 
OD,  OF,  and  OG  are  equal. 

At  the  moment  chosen  for  setting  out  the  clock  diagram  the 
voltage  magnitudes  of  A  and  B  are  equal,  and  therefore  each 


Electrode 
FIG.  55. — Nomenclature 
diagram. 


SINGLE  AND  POLYPHASE   CUEEENTS  TO   FUENACE  OPEEATION     63 


half  the  magnitude  of  C.  The  vertical  distances  of  the  points 
D,  F,  and  G  from  the  horizontal  axis  is  a  measure  of  the  mag- 
nitude and  sign  of  the  voltages  at  a  particular  moment,  and,  if 
the  radii  are  swinging  in  a  clockwise  direction,  the  magnitude 
of  the  voltage  in  the  circuit  A  is  falling,  that  in  B  rising,  and 
that  in  C  at  a  maximum  negative  value,  but  falling  towards 
zero.  The  radii  OD,  OF,  and  OG,  as  set  out  in  the  diagram, 


B 


0'      01    \02      F\   \03      G 


FIG.  56. 

graphically  fulfil  these  conditions,  and,  if  rotating,  may  be  as- 
sumed to  represent  the  constantly  varying  voltage  conditions  in 
the  three  circuits  A,  B,  and  C  (Fig.  57).  At  the  particular 
moment  shown  in  the  clock  diagram  it  is  assumed  that  the  in- 
stantaneous voltage  values  at  D,  F,  and  G  are  +  6,  +  6,  and 
-  12  relatively  to  the  terminals  O',  O2,  and  O3  respectively. 

The  impressed  voltages  in  the  circuits  A  and  B  both  have 
a  positive  sign  at  the  terminals  D 
and  F,  when  0'  and  O2  are  con- 
nected together ;  supposing  now 
the  terminal  O'  is  connected  to 
terminal  F,  it  is  obvious  that  it  is 
equivalent  to  reversing  the  sign  of 
the  voltage  impressed  in  the  cir- 
cuit B,  relative  to  the  circuit  A,  or, 
in  other  words,  to  reversing  the 
circuit  B  through  180°.  Therefore, 
if  B  were  originally  lagging  120°  behind  A,  it  will  now  lead  A 
by  60°,  and  the  radii  02F  and  O'D  may  be  now  shown  graphi- 
cally in  their  correct  relationship  by  the  lines  DO',  FO2  of  Fig. 
58,  assuming  always  the  radii  O'D,  FO2  to  be  rotating  in  a 
clockwise  direction. 

In  the  same  way,  when  the  terminals  O2  and  O3  are  connected 
together,  the  voltage  at  terminal  G  is  negative,  and  that  at  F 
positive ;  supposing  now  that  the  terminal  G  is  connected  to 


FIG.  58. 


64  THE    ELECTROMETALLURGY   OF    STEEL 

O2,  then  it  is  again  equivalent  to  reversing  the  sign  of  the  volt- 
age impressed  in  the  circuit  C,  relative  to  the  circuit  B,  and 
instead  of  lagging  120°  behind  B  it  will  now  lead  B  by  60°.  By 
drawing  a  line  GO3  from  the  point  O2,  to  represent  the  new  re- 
lationship of  the  circuits  C  and  B,  a  closed  triangle  will  be  formed 
which  represents  graphically  Delta  or  Mesh  grouping  of  three- 
phase  circuits. 

The  circuits  A  and  B  at  the  moment  considered  have  instan- 
taneous voltage  values  of  +  6  at  the  terminals  D  and  F,  and 
the  circuit  C  a  value  of  --  12  at  the  terminal  G ;  then;  on  con- 
sidering the  magnitude  and  direction  of  current  flow  through 
each  circuit  resulting  from  their  impressed  voltages,  it  is  evident 
that  the  circuits  A  and  B  together  oppose  and  neutralise  the 
effects  of  C.  This  applies  at  any  other  given  moment,  so  that 
three  circuits  generating  alternating  voltages,  similar  in  magni- 
tude and  frequency  but  differing  in  phase  by  120°,  may  be 
connected  to  form  a  closed  ring  circuit  without  causing  any  cur- 
rent to  flow.  If  three  line  wires  LY,  L2,  and  L3  are  taken  from 
the  common  terminals  DO3,  FO',  GO2,  and  a  resistance  r,  be 
connected  across  any  one  pair  L'  and  L2,  then  a  current  will 
flow  due  to  the  voltage  impressed  in  the  circuit  A.  This  also 
applies  to  any  other  resistance  circuit  connecting  other  pairs  of 
line  wires.  If  the  outside  resistances  between  each  pair  are 
equal,  then  the  effective  current  flowing  through  each  phase  A, 
B,  or  C  will  also  be  equal,  when  they  are  said  to  be  balanced. 
The  resistance  or  load  circuits,  as  distinct  from  the  generating 
circuits  A,  B,  and  C,  when  connected  across  the  line  wires  are 
themselves  "  mesh  "  connected. 

The  resistance  or  load  circuits  might  also  be  connected  in 
series  with  the  line  wires,  instead  of  across  them,  and  then  joined 
together  at  a  common  star  or  neutral  point.  This  case  is  analo- 
gous to  a  three-phase  direct  arc  furnace,  where  three  arcs  strike 
on  to  a  charge  or  bath  which  constitutes  a  star  point,  the  line 
wires  being  taken  from  three  mesh-connected  generating  circuits. 

With  this  combination  of  mesh  and  star  connections,  the 
effective  current  flowing  through  each  line  conductor  will  be 
1'73  times  the  current  flowing  through  each  generating  phase, 
provided  the  three  phases  are  balanced.  The  line  or,  in  this 
case,  the  generating  phase  voltage  will  be  1'73  times  the  voltage 


SINGLE   AND  POLYPHASE   CURRENTS  TO  FURNACE  OPERATION     65 

across  each  arc  or  other  similarly  grouped  power-absorbing 
circuit. 

Inverted  Star  Grouping. — It  has  been  shown  how  three- 
phase  circuits  may  be  connected  together  in  a  simple  star  group- 
ing, represented  graphically  by  three  equal  radii  120°  apart. 

Now  if  one  of  the  circuits  is  disconnected  from  the  neutral 
point,  reversed  and  again  connected  to  the  neutral  point,  it 
will  no  longer  be  120°  out  of  phase  with  the  other  two,  but 
will  lead  one  and  lag  the  other  by  60°.  This,  perhaps,  may 
be  more  simply  understood  by  reference  to  a  wave  form  dia- 
gram. In  Fig.  59  three  equal  alternating  voltages  120°  apart  are 
represented  by  their  wave  forms  A,  B,  and  C  ;  now,  supposing 
the  wave  form  C  is  reversed,  which  would  result  by  reversing 
the  connection  of  the  phase  C  relative  to  A  and  B,  then  the 
wave  C'  will  obviously  be  introduced  into  the  system  in  place 


JLJ. 


FIG.  59. 

of  C.     It  is  plain,  also,  that  the  phase  displacement  between  A 
and  C',  B  and  C'  is  60°. 

The  simple  inverted  star  connection  has  been  used  for  fur- 
nace operation,  but  in  its  more  recent  applications  a  special 
modification  is  adopted. 

Suppose  a  circuit  represented  by  the  wave  form  C,  after  dis- 
connection from  the  common  star  point,  were  split  into  two 
parts,  then  the  impressed  voltage  in  each  portion  would  be  in 
the  same  phase,  and  might  be  represented  by  two  separate  wave 
forms  of  different  magnitude,  whose  sum  exactly  equalled  the 
original  wave  form  C.  Either  component  might  be  again  star- 
connected  to  the  other  phases  A  and  B  so  as  to  be  120°  apart, 
while  the  other  component  might  be  reversed  before  connecting 
again  to  the  star  point. 

The  graphical  figure  would  then  be  a  combination  of  a 
simple  star  and  inverted  star  connection, 

5 


66  THE   ELECTRO-METALLURGY   OF    STEEL 

After  this  brief  description  of  the  commonest  methods  of 
compounding  three-phase  alternating  current  circuits,  their 
application  for  furnace  operation  may  now  be  considered. 

Stassano  Three= Phase  Furnace. — This  furnace,  being  de- 
pendent upon  indirect  arc  heating,  is  provided  with  three 
nearly  horizontal  electrodes,  which  converge  towards  a  common 
centre  and  are  set  120°  apart.  Arcs  are  struck  between  the 
electrode  ends,  and  form  an  equilateral  triangle  whose  apices  are 
the  electrode  points.  By  this  formation  of  the  arcs,  the  elec- 
trodes may  be  regarded  as  mesh  connected  at  that  end  where 
the  electrical  energy  is  being  converted  into  heat  in  the  arc 
gaps.  The  current  flowing  through  each  arc,  when  all  three 
circuits  are  balanced,  is  then  equal  to  the  current  flowing 

through  any  electrode  divided  by  T73,  so  that  the  load  in  K.V.A. 

^ 
taken  by  each  arc  =  ^=-~  x  V  -f-  1000 

1*  I  O 

where  A  =  current  flowing  through  any  electrode 

V  =  arc  voltage,  which  is  in  this  case  the  line  voltage 
The  total  load  in  the  furnace  is   thus  represented  by  the 
equation — 

K.V.A.  =  -4o  *  V  x  3  -  1000 
l/o 

provided  the  value  of  the  current  flowing  through  each  elec- 
trode is  the  same. 

Heroult  Three = Phase  System. — When  static  transformers 
are  used  for  supplying  power,  the  low  tension  windings  are 
usually  mesh  connected,  as  shown  in  Fig.  60.  They  may,  in 
certain  cases,  be  connected  to  form  a  "  star "  grouping 
(Fig.  54),  but  without  the  use  of  a  neutral  return  circuit  from 
the  furnace  hearth  to  the  "  star  "  point.  These  two 
methods  of  grouping  are  sometimes  made  interchangeable  by 
means  of  special  switchgear  for  the  purpose  of  effecting  a  con- 
siderable variation  of  line  voltage.  The  high  tension  windings 
are  equally  well  connected  in  either  "  star  "  or  mesh  grouping, 
which  can  likewise  be  made  interchangeable. 

The  outstanding  feature  of  the  Heroult  system  is  the  absence 
of  any  hearth  connection,  so  that  the  supply  of  energy  to  the 
furnace  charge  is  entirely  independent  of  the  linjng.  The  three 


SINGLE  AND  POLYPHASE    CURRENTS   TO  FURNACE  OPERATION     67 


arcs  strike  on  to  the  charge  or  bath,  which  serves  as  the  common 
star  point,  and  are  thus  star  connected.  When  the  electrode 
circuits  carry  the  same  current  the  total  load  on  the  furnace  in 
V 


KV.A. 


1-73 


x  A  x  3  -  1000 


where  A  =  current  flowing  through  the  electrodes 

V  =  line  voltage  or  voltage  between  any  two  electrodes. 

The  load  in  K.V-A.  taken  by  each  arc  is  equal  to  the  current 


FIG.  60. 


FIG.  61. 


flowing  multiplied  by  the  arc  voltage  and  divided  by  1000,  which  is 

Y 

represented  by  ^=»  x  A  -=-  1000  in  the  above  equation. 

Three -Phase  Four -Wire  System. — This  system  generally 
entails  the  use  of  a  bottom  electrode  connected  by  a  neutral 
return  conductor  to  the  star  point  of  the  supply  circuits,  as 
shown  in  Fig.  55,  and  has  been  used  by  Keller,  Girod,  and  Giffre 
in  various  modified  ways. 

Girod  Three -Phase  Inverted  Star  Connection. — This  method 
of  connecting  the  three  supply  circuits  is  shown  in  Fig.  61,  where 
it  will  be  seen  that  one  phase  of  a  simple  star-connected  system 


68  THE    ELECTEO-METALLUEGY   OF    STEEL 

has  been  reversed.  Girod  adopted  this  method  in  order  to 
increase  the  current  flowing  through  the  bottom  electrodes.  It 
is  very  doubtful  whether  this  was  a  wise  step,  and  it  will  be  seen 
later  how  the  modern  tendency  is  rather  to  restrict  the  current 
flowing  through  the  furnace  hearth  without  unbalancing  the 
supply  phases; 

Stobie  Three= Phase  Four=Wire  System. — The  simple  case 
of  a  four- wire  three-phase  grouping  has  already  been  mentioned, 
where  the  neutral  return  was  connected  between  a  bottom 
electrode  and  the  star  point.  According  to  Stobie's  method  the 
neutral  return  is  taken  from  the  star  point  and  connected  to  an 
upper  electrode  similar  in  mechanical  operation  and  dimension 
to  the  other  three  electrodes  placed  in  the  line  conductor  circuits. 
This  fourth  neutral  electrode  carries  any  out-of-balance  current 
when  the  three  main  electrodes  are  unbalanced,  but  is  other- 
wise inoperative. 

Greaves- Etchells  Three= Phase  Systems. — In  all  the  pre- 
viously mentioned  methods  of  supplying  direct  arc  furnaces  with 
power  from  three-phase  circuits,  with  equal  line  voltages,  an 
electrode  has  been  interposed  in  each  line  conductor  so  as  to 
produce  three  arcs  star  connected  to  a  common  point,  which  is 
the  metallic  charge  or  bath  of  metal.  It  has  also  been  shown 
that  the  power  supply  circuits  are  only  balanced  when  such 
a  star-connected  load  is  equally  divided  between  each  line 
resistance  or  arc.  With  three  arcs  the  load  can  be  balanced 
by  adjustment  of  the  arc  length,  but  if  the  resistance  of  one 
arc  is  fixed,  then  balance  could  only  be  obtained  by  making  the 
resistance  of  each  of  the  other  two  equal  to  it.  In  the  Greaves- 
Etchells  furnace  only  two  electrodes  are  used  for  the  purpose 
of  forming  arcs,  and  one  fixed  electrode  is  imbedded  in  a  hearth 
of  variable  resistance.  The  hearth  resistance  may  be  regarded 
as  being  substituted  for  one  arc  resistance,  but,  instead  of  being 
adjustable,  is  dependent  solely  upon  the  conductivity  of  the 
material  used  for  its  construction  at  different  temperatures. 
The  essence  of  all  the  Greaves-Etchells  patents  lies  in  the 
special  methods  of  transformer  design  and  grouping,  so  that 
a  balanced  load  can  be  obtained  on  the  primary  supply  phases 
when  the  hearth  resistance  does  not  necessarily  equal  the  arc 
resistances.  Provision  is  also  made  so  that  these  conditions  of 


SINGLE  AND    POLYPHASE   CURRENTS  TO   FURNACE  OPERATION     69 

balance  may  be  possible  over  a  wide  variation  of  hearth  resist- 
ance, provided  that  each  arc  carries  the  same  current. 

A  full  description  of  the  several  methods  of  transformer 
grouping  possible  is  beyond  the  scope  of  this  book,  as  it  can  only 
be  comprehensibly  studied  by  the  use  of  complex  vector 
diagrams. 

Application  of  Four = Phase  Currents. — In  Fig.  62  are  repre- 
sented the  wave  curves  A  and  B  of  two  single-phase  currents 
90°  out  of  phase.  The  curve  A  may  be  split  up  into  two  equal 
components  C  and  C',  which,  owing  to  their  equality,  are  super- 
imposed on  the  diagram.  Supposing  terminal  connections  are  so 
made  that  C'  is  reversed  relatively  to  C,  then  there  will  be  two 
single-phase  waves  180°  out  of  phase,  and  if  the  same  be  applied 
to  the  wave  B  to  form  two  components  D  and  D',  of  which  the 
latter  is  reversed,  there  will  then  be  four  curves  90°  apart, 


FIG.  62. 

arranged  in  the  order  C,  D,  C',  and  D'.  If  these  four  circuits 
were  star  connected,  they  could  be  represented  in  a  clock 
diagram  by  equal  radii  90°  apart,  which  would  denote  all  the 
relations  of  one  phase  to  another.  It  is  quite  obvious  that  the 
algebraic  sum  of  the  impressed  E.M.Fs.  at  any  moment  is  zero, 
so  that,  if  the  four  individual  phases  were  connected  in  ring 
fashion,  just  as  in  the  case  of  three-phase  circuits,  no  current 
would  flow.  If,  however,  any  outside  circuit  were  connected 
across  any  two  points  on  the  periphery  of  the  ring,  then  a 
current  would  flow  through  that  circuit. 

The  earlier  types  of  two-phase  furnaces  were  designed  for 
small  capacities  up  to  about  five  tons,  and  for  this  size  two  upper 
electrodes,  acting  in  conjunction  with  one  or  more  bottom 
electrodes,  are  sufficient  to  convey  the  required  amount  of 
electrical  energy  to  the  furnace.  With  the  growing  tendency 


70 


THE   ELECTEO-METALLUEGY   OF    STEEL 


towards  larger  units,  it  has  become  imperative  to  increase  the 
number  of  upper  carbon  conductors,  with  the  logical  introduc- 
tion of  four-phase  currents.  It  has  already  been  seen  how  two- 
phase  currents  could  be  applied  in  the  case  of  four  upper 
electrodes  without  a  neutral  return,  and  it  now  remains  to 
describe  other  methods  by  which  two-phase  currents,  which 
have  been  compounded  into  four-phases  by  splitting  each  phase 
and  reversing  one-half  of  each,  can  be  utilised  in  either  mesh  or 
star  grouping  (with  or  without  neutral  return  conductors). 

All  designers  of  bottom-electrode  furnaces  have  appreciated 
the  importance  of  varying  the  return  current  without  unbalanc- 
ing the  primary  phases,  and  it  will'be  seen  how  this  feature  has 


FIG.  63. 

been  introduced  into  some  of  the  systems  of  four-phase  group- 
ing. 

Dixon's  Four-Phase  Star  Grouping.1 — The  diagram  of  con- 
nections (Fig.  63)  shows  a  simple  star  grouping  of  four-phase 
circuits,  the  "  star  "  point  being  connected  by  a  neutral  return 
conductor  to  a  bottom  electrode  embedded  in  a  conductive 
hearth.  The  four-phase  current  is  derived  from  the  two-phase 
circuits  which  form  the  secondary  windings  of  two  groups  of 
Scott-connected  transformers.  By  means  of  switches  in  the 
three-phase  primary  line  conductors  the  two-phase  secondary 
windings  of  one  transformer  group  can  be  made  either  180°  or 
120°  out  of  phase  with  the  other.  In  the  first  case  the  four- 
phase  circuits  are  90°  out  of  phase,  and  no  current  will  flow 

British  Patent  Specification,  No.  4742,  A.D.  1914. 


SINGLE  AND  POLYPHASE   CURRENTS    TO  FURNACE    OPERATION     71 

under  balanced  loading  through  the  neutral  return.  Various 
other  modifications  have  been  introduced  for  further  varying 
the  phase  relationship  of  one  pair  of  circuits  relatively  to  the 
other,  this  being  done  for  the  purpose  of  varying  the  current 
flowing  through  the  furnace  bottom,  while  still  maintaining  a 
balanced  loading  on  the  primary  phases.  The  system  of  con- 
nections shown  in  Fig.  63  is  used  on  a  Gronwall-Dixon  5-ton 
furnace  operating  at  Detroit,  U.S.A. 

Dixon's  Four-phase  Mesh  Grouping. — This  method  is  far 
less  complicated  than  the  above  and  consists  of  a  simple  ring 
connection  shown  graphically  in  the  form  of  a  square  (Fig.  64) 
From  each  corner  is  branched  off  a  conductor,  which  conveys 
current  to  an  electrode.  The  load  on  the  supply  phases  is 


JIG.  64. 

balanced  when  the  current  flowing  through  each  electrode  is 
equal.  This  system  may  be  regarded  as  the  four-phase  counter- 
part of  the  simple  Heroult  three-phase  mesh  connection. 

Electro= metals  Four= phase  Five- wire  Grouping. — This 
system  of  connections  embodies  a  special  arrangement  of  the 
three-phase  secondary  windings  of  a  transformer  group.  Five 
line  conductors  are  required,  four  of  which  are  connected  to 
upper  electrodes  and  one  to  a  conductive  hearth.  It  is  also  a 
feature  of  this  method  that  the  bottom  electrode  conductor  takes 
only  little  more  current  than  any  one  of  the  upper  electrodes 
under  balanced  loading.  A  full  explanation  of  the  manner  by 
which  the  above  conditions  are  accomplished  is  only  possible 
by  resorting  to  complicated  vectorial  or  mathematical  solutions, 
so  that  it  is  only  proposed  here  to  convey  the  general  principle, 


72 


THE    ELECTBO-METALLTJKGY   OF   STEEL 


and  to  point  out  the  fundamental  difference  between  this  and 
the  other  methods  of  grouping  previously  described. 

The  three  secondary  windings  are  split  up  into  parts,  and 
are  connected  to  four  upper  electrodes  and  a  bottom  electrode 
in  the  manner  shown  by  Fig.  65.  AB,  CD,  and  EF  represent 

the  three-phase  secondary  circuits  of  a 
transformer  group.  The  transformer 
windings  AB,  CD,  and  KF  are  re- 
spectively tapped  at  the  points  G,  H, 
and  I.  The  points  G  and  H  are  con- 
nected to  opposite  ends  of  the  wind- 
ings EF,  and  the  point  I  is  connected 
to  the  bottom  electrode  5.  A  and  B 
are  connected  to  upper  electrodes  3 
and  4,  and  C  and  D  to  2  and  1  respec- 
tively. It  is  clear  that  the  voltage  be- 
tween the  bottom  electrode  or  the  point 
I  and  any  one  upper  electrode  is 
always  the  resultant  of  the  voltages 

induced  in  one  section  of  either  winding  AB  and  CD,  and 
one-half  of  the  winding  FE.  The  wave  curve  representing 
the  voltage  between  electrodes  1  and  5  is  the  resultant  of  the 
wave  curves  of  the  impressed  voltages  across  the  windings 
HD  and  El,  which  are  different  in  magnitude  and  120°  out  of 
phase.  In  Fig.  66  the  curves  A  and  B,  120°  apart,  represent 


FIG.  65. 


FIG.  66. 

the  impressed  voltages  across  these  windings  El  and  HD  ;  curve 
C  is  drawn  by  plotting  points  whose  distances  above  or  below 
the  line  XY  are  equal  to  the  algebraic  sum  of  the  vertical  dis- 
tances of  the  curves  A  and  B  from  X  Y  at  several  given  moments. 
The  curve  C  represents  the  resultant  of  curves  A  and  B,  both 
as  regards  magnitude  of  impressed  E.M.F.  and  phase.  Since 


SINGLE  AND  POLYPHASE  CURRENTS  TO  FURNACE  OPERATION     73 

there  are  four  distinct  electrode  circuits  built  up  of  two  unequal 
windings  in  series,  and  whose  phase  relationships  are  in  every 
case  different,  there  will  be  four  resultant  curves,  and,  therefore, 
a  four-phase  system,  in  which,  however,  the  phases  are  not  90° 
apart,  as  is  usually  the  case.  The  magnitude  of  each  resultant 
voltage  curve  must  be  the  same,  otherwise  the  load  cannot  be 
properly  balanced,  and  it  is  for  this  reason  that  the  windings 
AB  and  CD  are  both  unequally  divided. 

In  the  diagram  the  points  E  and  H  are  connected  together, 
and  the  voltage  curve  of  El  will  then  be  120°  in  advance  of 
HD,  assuming  El  and  HD  to  be  moving  about  a  centre  H  in  a 
clockwise  direction,  this  being  the  convention  followed  through- 
out. Now  El  and  HC  are  similarly  connected,  but  in  this  case 
HC  is  60°  in  advance  of  El,  which  is  evident  since  the  voltage 
induced  in  HC  is  exactly  opposite  in  sign  to  that  induced  in 
HD  in  their  relationship  to  EL  The  relative  phase  displace- 
ments of  the  four  resultant  curves  are  unimportant,  so  long  as 
their  magnitude  is  equal.  Since  the  phase  displacement  between 
the  windings  HD,  El,  and  HC,  El  are  different,  then  the  only 
way  in  which  the  magnitude  of  the  resultant  can  be  made  the 
same  is  by  making  the  voltages  across  HD  and  HC  different, 
and  to  effect  this  both  windings  AB  and  CD  are  unequally 
divided. 

By  this  method  of  grouping  the  secondary  windings  of  a 
three-phase  system  can  be  so  arranged  that  the  voltages  between 
four  electrodes  and  a  neutral  bottom  are  similar,  and  the  current 
through  the  neutral  return  is  nearly  equal  to  the  current  through 
each  electrode  when  balanced. 


CHAPTEE  IV. 

GENEEATION  AND  CONTROL  OF  SINGLE  AND  POLYPHASE 
CURRENTS. 

Single -phase  Installations. — The  early  types  of  electric  fur- 
naces operated  on  single-phase  current  supply,  and  usually  de- 
rived their  power  from  alternators  attached  to  individual  units. 
Power  could  equally  well  be  taken  from  single-phase  supplies 
generated  at  extra  high  voltage,  and  transformed  down  by  static 
transformers.  In  this  latter  case,  if  the  normal  furnace  load  is 
only  a  fraction  of  the  total  power  generated  in  the  system,  a 
heavy  load  fluctuation  will  not  be  accompanied  by  any  serious 
diminution  of  the  line  voltage,  and,  provided  the  static  trans- 
former is  suitably  designed,  heavy  overloads  may  occur 
without  endangering  any  part  of  the  electrical  installation. 
This,  however,  does  not  apply  to  alternators  separately  con- 
nected to  furnace  units,  and  special  provision  has  then  to  be 
made  to  prevent  heavy  sudden  overloads  which  might  prove 
disastrous,  not  only  to  the  alternator,  but  also  to  its  prime 
mover. 

Single-phase  current  is  seldom  generated  by  Power  Com- 
panies for  local  consumption  in  high  power  machinery,  or  for 
long  distance  transmission  other  than  for  traction  purposes. 
For  this  reason  it  is  generally  necessary  to  install  special 
generating  plant  for  single-phase  furnace  operation,  since  poly- 
phase currents  cannot  be  satisfactorily  transformed  to  single- 
phase  by  static  transformers.  This  adds  considerably  to  the 
capital  cost  of  the  installation,  and,  where  motor-generator  sets 
are  used,  a  total  loss  of  about  15  percent,  of  the  power  consumed 
is  incurred.  The  problem  of  supplying  power  to  single -phase 
furnaces  stands  out  as  a  predominant  objection  to  their  use, 
unless,  as  is  rarely  the  case,  the  Power  Supply  Co.  allows  the 

(74) 


GENEEATION    OF    SINGLE   AND   POLYPHASE    CURRENTS         75 

furnace  load  to  be  taken  from  separate  phases  of  a  polyphase 
system. 

Single -phase  Generators. — Alternators  and  their  prime 
movers,  supplying  power  to  separate  furnaces,  are  usually  rated 
at  normal  full  load  capacity,  and  must  be  designed  to  prevent, 
automatically,  heavy  overloads  which  might  otherwise  prove  a 
constant  source  of  breakdown.  The  power  absorbed  in  an 
alternating  current  circuit  is  the  product  of  volts  and  amperes 
(volt-ampere)  multiplied  by  the  power  factor,  and  a  variation  of 
any  one  of  these  multiples  will  cause  a  change  in  the  effective 
power.  It  has  been  explained  how  the  effect  of  the  self-induc- 
tion of  a  circuit,  which  depends  partly  on  the  strength  of  the 
current  flowing,  produces  an  opposing  E.M.F.  or  voltage  and 
at  the  same  time  causes  a  lowering  of  the  power  factor,  and  it 
is  this  very  property  that  is  utilised  in  the  design  of  generators 
supplying  current  subject  to  sudden  and  heavy  fluctuation. 
The  alternator  is  constructed  to  have  a  "  drooping  character- 
istic," which  denotes  a  rapid  falling  off  of  the  terminal  voltage 
so  soon  as  the  current  in  the  circuit  rises  above  the  normal ; 
the  power  capable  of  being  developed  is  thereby  automatically 
restricted  by  a  reduction  of  the  voltage,  although  the  actual 
current  will  rise,  but  not  to  the  same  extent  as  if  the  normal 
line  voltage  were  maintained.  An  exciting  dynamo  is  generally 
mounted  on  the  generator's  shaft  and  is  itself  excited  from  an 
independent  source,  or  it  may  be  self -excited.  The  excitation 
current  for  the  field  windings  of  the  alternator  is  controlled  by 
varying  the  strength  of  current  flowing  through  the  field  of 
the  small  exciter,  and  by  this  means  the  alternator  may  be 
regulated  to  maintain  a  constant  voltage  over  a  range  of  different 
current  outputs.  In  effect  then,  the  alternator  may  be  regulated 
to  supply  power  at  the  same  voltage  for  any  desired  current 
within  its  capacity,  any  increase  above  this  current  giving  rise  to 
a  drop  of  voltage.  Eegulation  of  the  furnace  load  is  effected  by 
maintaining  the  correct  terminal  voltage,  which  is  done  by 
adjustment  of  the  electrodes.  It  is  also  customary  to  provide 
the  generating  set,  whether  the  prime  mover  be  an  electric 
motor,  steam  or  gas  engine,  with  a  considerable  flywheel  effect, 
which,  by  its  capacity  for  storing  or  giving  out  energy,  greatly 
minimises  the  result  of  load  fluctuations  upon  the  prime  mover. 


76 


THE    ELECTEO-METALLUKGY   OF    STEEL 


Miles-Walker  Converter. — A  special  type  of  three-phase  to 
single-phase  converter  has  recently  been  introduced  for  furnace 
operation  by  Professor  Miles- Walker,  which  takes  the  form  of 
a  rotating  balancing  transformer.  The  converter  takes  H.T. 
power  from  a  three-phase  supply  at  unity  power  factor,  and  de- 
livers L.T.  single-phase  current  without  unbalancing  the  supply 
phases.  As  in  the  case  of  single-phase  alternators,  the  balancer  is 
designed  to  have  very  considerable  self-induction  in  the  secondary 
or  low  tension  circuit,  so  that  at  normal  full  load  the  current 
curve  will  lag  45°  behind  the  voltage  curve,  giving  a  low  power 
factor  of  '7.  With  this  arrangement  it  is  impossible  on  dead 


0, 
0 
O 


Oo 
C) 
O 


1 

t.  i 
<u 

^ 

£ 


1000 


2000 
Amperes 

FIG.  67. 


$000 


4000 


short  circuit  to  obtain  a  current  greater  than  1'41  times  the 
normal  full  load  value,  and  this  current  rise  is  accompanied  by 
an  actual  falling  off  of  the  power  output.  The  single-phase 
terminal  voltage  will  steadily  decrease  when  increasing  the  load 
from  open  circuit  to  normal  full  load,  after  which  it  very  rapidly 
falls  away.  The  load  during  operation  may  be  varied  up  to, 
but  not  beyond,  full  load  current  by  lengthening  or  shortening 
the  arc. 

The  efficiency  of  transformation  is  90  per  cent,  showing  a 
gain  of  about  5  per  cent,  over  motor  generators.  The  curve 
shown  in  Fig.  67  shows  for  a  specific  case  how  the  power  output 


GENEEATION    OF   SINGLE-  AND   POLYPHASE    CURRENTS          77 

falls  off  as  soon  as  a  full  load  current  of  2700  amperes  is  exceeded. 
These  balancers  have  already  been  in  operation  in  England,  and 
should  prove  a  great  advance  on  the  motor  generator  sets  pre- 
viously used. 

Single-phase  Static  Transformers. — Single-phase  static  trans- 
formers are  almost  universally  used  in  furnace  installations  for 
transforming  the  high  tension  voltage  down  to  a  voltage  suitable 
for  furnace  use,  and  are  used  either  singly  or  grouped  together 
for  operation  on  single  or  polyphase  supplies  respectively. 

Power  transformers  for  furnace  work  require  to  be  specially 
designed  to  meet  the  severe  conditions  of  service,  and  the  fol- 
lowing remarks  upon  their  construction  apply  to  single-phase 
transformers  used  either  singly  or  grouped  together  for  a  poly- 
phase supply. 

Internal  Eeactance. — Direct  arc  furnaces,  when  melting 
cold  scrap,  are  subject  to  momentary  overloads  which  may 
even  at  times  approach  a  dead  short  circuit,  and  under  such 
conditions  violent  surging  is  likely  to  be  set  up  unless  special 
provisions  are  taken  to  prevent  it.  There  is  also  consider- 
able risk  of  the  transformer  windings  becoming  displaced  by 
powerful  magnetic  forces  set  up  by  heavy  current  overloads, 
and  when  this  happens  breakdown  of  the  insulation  is  likely  to 
occur. 

It  has  been  previously  explained  how  the  self-induction  of  a 
circuit  acts  as  a  limiting  factor  to  the  degree  of  sudden  current 
variation,  so  that  by  suitable  design  the  degree  of  current  fluctua- 
tion and  its  harmful  effect  may  be  considerably  reduced.  To 
effect  this  transformers  are  usually  designed  with  8  per  cent, 
to  15  per  cent,  internal  reactance,  which  is  introduced  by  allow- 
ing flux  leakage  in  the  main  magnetic  circuit.  The  internal 
reactance  of  a  transformer  is  regarded  as  the  percentage  ratio 
of  reactive  volts,  at  normal  full  load,  to  the  impressed  volts. 

Heating. — The  fluctuating  character  of  a  furnace  load  pre- 
cludes the  use  of  any  automatic  device  which  will  open  the 
switch  on  other  than  heavy  overloads  sustained  for  a  period  of 
one  second  or  more.  Nothing,  therefore,  apart  from  the  care  of 
f  urnacemen,  can  prevent  a  small  sustained  overload  on  the  trans- 
formers, which  may  be  anything  up  to  15  per  cent,  or  even  more 
over  short  periods.  This  demands  careful  attention  from  the 


78  THE    ELECTKO-METALLUEGY   OF    STEEL 

power  transformer  designers,  who  usually  make  an  ample 
allowance  for  steady  overloads  in  calculating  the  temperature 
rise  under  ordinary  conditions  of  service.  The  air-cooled  oil- 
immersed  type  of  transformer  is  generally  favoured,  but  for  the 
largest  units  water  cooling  has  been  more  often  used. 

The  low  tension  copper  bars  are  frequently  interleaved 
where  they  emerge  from  the  iron  case,  and  extend  at  least  18" 
away  from  the  nearest  point  of  the  shell ;  any  heavy  mesh 
connections  can  then  be  safely  made  between  the  transformer 
terminals  without  fear  of  local  heating  in  the  steel  shell. 

Automatic  tripping  devices  are  sometimes  used  in  conjunc- 
tion with  mercury  thermometers,  which  are  immersed  in  the 
transformer  oil,  and  complete  relay  circuits  when  the  temperature 
exceeds  the  recognised  limit  of  safety.  There  can  be  no  doubt 
that  breakdowns  are  frequently  due  to  prolonged  overheating, 
which  in  course  of  time  carbonises  the  oil  and  causes  deterioration 
of  the  insulation,  so  that  any  safety  device,  which  might  at  least 
be  used  to  effect  a  warning,  is  a  useful  accessory  in  transformer 
equipments. 

Primary  Tappings. — In  all  direct  and  indirect  arc  furnaces 
the  arc  voltage  is  to  a  great  extent  limited  by  the  damage 
likely  to  result  to  the  refractory  lining  from  an  exposed  arc  of 
considerable  length.  In  the  case  of  direct  arc  furnaces  in  which 
cold  scrap  is  melted  and  refined,  the  arc  will  strike  between 
the  nose  of  the  electrode  and  the  charge  of  scrap  or  pool  of 
melted  metal,  and  at  the  same  time  the  heat  of  the  arc  will  be 
localised  and  shielded  from  the  walls  and  roof  by  a  surrounding 
wall  of  unmelted  scrap.  Therefore,  so  long  as  scrap  remains 
unmelted  above  the  bath  level,  a  higher  voltage  is  permissible 
than  when  the  entire  charge  is  melted  and  the  lining  con- 
sequently more  directly  exposed  to  the  arc. 

There  are  certain  advantages  to  be  gained  by  melting 
at  a  high  voltage,  so  that  it  is  preferable  to  work  at  different 
voltages  during  the  melting  down  and  subsequent  stages  of  the 
process.  There  are  various  ways  of  doing  this,  but  the  method 
most  generally  adopted  is  to  change  the  connections  of  the 
primary  windings  of  the  transformer  (see  Figs.  70  and  71). 
These  windings  are  tapped  at  suitable  points  and  connected  to 
separate  terminals;  by  changing  the  connections  between  the 


GENERATION    OF    SINGLE   AND   POLYPHASE    CURRENTS         79 

high  tension  supply  cables  and  these  primary  terminals  it  is 
possible  to  alter  the  transformation  ratio  and  thereby  obtain 
different  terminal  voltages  on  the  secondary  side.  The  number 
of  tappings  is  usually  restricted  to  two,  which  enables  either 
two  or  three  different  voltages  to  be  obtained  by  the  use  of 
simple  change-over  switches. 

There  are  two  methods  of  arranging  tappings  for  changing 
the  transformation  ratios,  the  usual  method  being  to  open  cir- 
cuit a  number  of  the  coils  adjacent  to  one  end  of  the  primary 
windings,  and  thus  reduce  the  number  of  turns.  When  the 
voltage  variation  is  small,  the  number  of  turns  eliminated  will 
be  few,  so  that  the  point  from  which  the  tapping  is  taken  is 
close  to  one  end  of  the  primary  windings.  Hill  and  Fleming 
have  pointed  out  that  at  the  moment  of  switching  in  a  trans- 
former there  is  likely  to  be  a  concentration  of  voltage  between 
the  adjacent  turns  of  the  primary  windings  near  the  ends  con- 
nected to  the  line  conductors,  so  that  in  cases  where  the  tappings 
are  taken  out  adjacent  to  one  end  of  the  windings  there  will  be 
a  high  potential  across  the  terminals  of  the  open  circuited 
portion,  which  may  tend  to  cause  a  breakdown  of  the  insulation. 
To  overcome  this  risk,  the  primary  windings  are  sometimes 
split,  and  tappings  are  taken  out  from  near  the  inner  end  of 
each  half.  By  suitable  switchgear  two  small  sections  of  the 
split  windings  are  cut  out  in  place  of  one  large  section  as  in  the 
more  usual  way.  The  switchgear  for  effecting  the  necessary 
connections  is  more  complex  than  the  simple  change-over  switch 
commonly  in  use  with  the  system  of  tapping  connection  first 
mentioned. 

Transformer  Grouping. — The  majority  of  modern  furnaces 
operate  on  a  two-phase  or  three-phase  low  tension  supply,  either 
of  which  can  be  obtained  by  a  simple  grouping  of  single-phase 
transformers  on  either  two-phase  or  three-phase  high  tension 
systems. 

Certain  special  methods  of  grouping  and  splitting  up  the 
transformer  circuits,  peculiar  to  particular  furnaces,  have  been 
already  dealt  with. 

Three-phase  to  Three-phase. — The  primary  windings  of  each 
single-phase  transformer  may  be  connected  together  either  in 
star  or  mesh  grouping,  which  likewise  applies  to  the  secondary 


80 


THE   ELECTRO-METALLURGY   OF    STEEL 


windings  ;  star  connections  of  both  primary  and  secondary  should, 
however,  be  avoided. 

Two-phase  to  Three-phase. — Two-phase  current  is  generally 
converted  to  three-phase  by  the  Scott  method  of  transformer 
grouping.  Two  single-phase  transformers  are  used  in  which 
the  primary  windings  consist  of  the  same  number  of  turns,  the 
secondary  windings  being  in  the  ratio  of  100  to  86'6.  Fig.  68 
shows  the  connection  of  two  transformers  made  so  that  one  end 
of  the  secondary  winding  AB  of  the  transformer  "X"  is  con- 
nected to  the  middle  point  of  the  secondary  winding  of  the 
transformer  "  Y  ".  The  resultant  line  voltage  between  A  and  D 
and  A  and  C  will  be  equal  to  the  line  voltage  across  DC  ;  in 
this  manner  three  independent  circuits  are  available  in  which 


wwwwJ     WwwwvJ 

/ww\  /ww\ 


FIG.   68. 


FIG.  69. 


the  impressed  voltages  are  equal  in  magnitude  and  are  120° 
apart. 

Three-phase  to  Two-phase. — This  case  is  merely  the  reverse 
of  the  above,  the  transformation  being  accomplished  by  design- 
ing the  windings  AB  and  DBG  for  the  high  tension  primary 
circuit  in  place  of  the  secondary,  the  ratio  of  turns  remaining 
exactly  the  same.  The  usual  diagrammatic  method  of  indicating 
a  Scott-connected  transformer  group  is  shown  in  Fig.  69  ;  this 
method  avoids  the  confusion  of  crossed  high  and  low  tension 
line  conductors. 

Voltage  Variation  :  Transformer  Tappings. — The  provision 
of  tappings  taken  from  the  primary  circuits  of  transformers  has 
already  been  mentioned  as  a  means  of  altering  the  transforma- 
tion ratio  arid  the  secondary  voltage. 

The  method  of  changing  the  connections  between  the  tap- 


GENERATION    OF    SINGLE   AND   POLYPHASE    CURRENTS         81 


pings  and  the  supply  cables  by  means  of  a  special  switch 
depends  upon  whether  the  primary  windings  are  star  or  mesh- 
connected.  Fig.  70  shows  a  three-phase  transformer  group 
mesh-connected  to  the  high  tension  supply,  and  the  method 
adopted  for  changing  the  secondary  voltage  by  means  of  a  simple 
two-way  knife  switch.  If  a  greater  range  is  required,  a  more 
complex  switch  must  be  used,  but  generally  two  voltages  are 
quite  sufficient  for  practical  purposes.  In  Fig.  71  is  shown  the 
arrangement  when  the  primary  circuits  are  star-connected  ;  a 
special  switch  is  required  which  enables  different  sets  of  tappings 
to  be  starred,  and  is  exceedingly  simple  in  construction.  Change 


t 

1 

1 

~J_ 

1 

.A/W/V 

r  r 

.    f 

1 

FIG.  70. 


FIG.  71. 


voltage  switches  are  always  carefully  interlocked  with  the  main 
oil  switch,  so  that  on  no  account  can  the  primary  connections 
be  altered  when  the  transformers  are  "  alive  ". 

By  the  use  of  tappings  the  range  of  voltage  change  is  usually 
small,  and,  where  a  greater  variation  is  required,  recourse  is  made 
to  alteration  of  the  transformer  grouping  on  the  primary  circuits. 
This  entails  a  change  from  "  star  "  to  "  mesh  "  grouping,  or  vice 
versa,  which,  of  course,  can  only  be  employed  on  three-phase 
systems.  If  the  primary  windings  of  three  single-phase  trans- 
formers are  mesh-connected,  and  each  has  a  transformer  ratio 
of  11,000  to  100,  then  the  voltage  across  each  secondary  wind- 
ing will  be  100  for  an  11,000  volt  system.  Now  supposing  the 

6 


82 


THE    ELECTRO-METALLUBGY   OF    STEEL 


primary  transformer  circuits  are  "  star  "  connected,  then  the 
voltage  across  each  primary  winding  will  be  only  11,000  -v-  .^8 
or  6360,  and  the  voltage  across  each  secondary  57*8,  so  that  in 
this  way  the  low  tension  voltage  can  be  reduced  in  the  same 
ratio  of  T73  to  1.  The  KV.A.  capacity  of  the  transformer  will 
at  the  same  time  be  reduced  in  proportion  to  the  voltage  across 
each  primary  circuit,  but  this  is  seldom  a  serious  disadvantage, 
as  the  high  voltage  arrangement  of  the  transformers  would  only 
be  used  during  the  melting  operation,  and  the  lower  voltage  for 
subsequent  refining  when  the  amount  of  power  then  required  is 
obtainable  with  the  lower  voltage  grouping.  This  reduction  of 
available  power  is  unavoidable,  since  the  transformer  windings 
must  not  be  overloaded  to  counterbalance  the  voltage  reduction. 


FIG.  72. 

Eeactance  Coils, — In  certain  types  of  furnace  installations 
large  reactance  coils  are  introduced  in  either  the  primary  or 
secondary  transformer  circuits  for  the  purpose  of  eliminating 
load  fluctuation.  In  such  cases  the  reactive  voltage  drop  (at 
normal  full  load  current)  is  about  70  per  cent,  of  the  line  voltage, 
and  the  large  reactance  coils  necessary  for  this  purpose  can  be 
conveniently  used  to  obtain  a  small  range  of  voltage  variation 
at  the  furnace  terminals  for  a  given  full  load  current.  This  is 
done  by  varying  the  number  of  the  windings  of  the  reactance 
coil,  according  to  the  arrangement  shown  diagrammatically  in 
Fig.  72.  In  this  case  a  number  of  tappings  T  are  taken  off  the 
windings,  any  three  of  which  may  be  permanently  connected  to 
leads  L  passing  to  a  selector  switch  S.  This  arrangement  en- 
ables the  reactive  resistance  in  the  primary  circuit  to  be  varied, 
so  that  the  actual  voltage  across  the  terminals  of  the  furnace  is 


GENERATION    OF   SINGLE   AND   POLYPHASE    CURRENTS         83 

• 

capable  of  variation.  Here  again  a  reduction  of  voltage  results 
in  reduced  power  since  the  current  must  be  kept  below  a  fixed 
maximum.  This  method  of  voltage  control  is  not  applicable, 
except  in  cases  where  large  reactance  coils  are  purposely  intro- 
duced for  controlling  load  fluctuation. 

Reactance  or  Choking  Coils. — A  reactance  coil  may  be  re- 
garded as  a  device  for  introducing  or  increasing  interlinkings  of 
magnetic  lines  of  force  with  a  circuit.  It  has  been  explained  in 
Chapter  II.  that  a  reactive  voltage  is  self-induced  in  a  coiled  con- 
ductor carrying  an  alternating  current  by  virtue  of  the  constantly 
varying  magnetic  fields  set  up.  Also,  by  increasing  the  strength 
of  the  magnetic  field,  or  by  increasing  the  number  of  turns 
interlinking  the  magnetic  field,  the  reactive  voltage  will  be 
increased  if  the  current  flowing  and  the  frequency  remain 
unchanged. 

A  reactance  or  choking  coil  simply  consists,  then,  of  a  number 
of  turns  of  a  conductor,  sometimes  enclosing  a  laminated  iron 
core  which  may  be  either  closed  to  form  a  ring  or  may  be  broken 
to  include  an  air  gap.  The  windings  of  the  reactance  coil  are 
connected  in  series  with  the  circuit  in  which  the  reactance  is 
required  to  be  introduced,  and  therefore  form  an  integral  part 
of  the  circuit  itself.  The  various  power  circuits  of  any  arc  fur- 
nace will  include  the  simple  resistance  of  the  conductors,  the 
practically  non-inductive  arc  resistance,  and  a  reactive  resistance. 
The  applied  voltage  across  the  circuits  must,  then,  be  sufficient 
to  overcome  the  reactive  voltage  and  to  force  the  current  through 
the  non-inductive  resistance.  Since  the  wave  curves  of  the  re- 
sistance volts  (i.e.  the  volts  forcing  the  current  through  the  non- 
inductive  resistance,  and  therefore  in  phase  with  the  current) 
and  reactive  volts  are  90°  out  of  phase,  the  applied  volts  will  not 
be  the  simple  but  the  vectorial  sum  of  the  two.  Since  the  effect 
of  a  reactance  coil  depends  upon  an  increase  of  the  voltage  drop 
caused  by  current  overloads,  it  is  important  in  their  design  to 
study  the  reactive  drop  at  full  load,  and  under  heavy  overloads. 
Except  in  special  cases  the  voltage  drop  at  full  load  is  preferably 
small  but  increases  rapidly  under  current  overloads.  To  secure 
this  effect  the  iron  core  or  the  magnetic  circuit  must  be  so  de- 
signed that  the  number  of  lines  of  force  are  increased  propor- 
tionately to  the  current  flowing  ;  this  is  only  possible  so  long  as 


84 


THE    ELECTRO-METALLUKGY   OF    STEEL 


the  iron  circuit  remains  unsaturated.  The  effect  of  a  reactance 
coil  so  constructed  depends  upon  the  ratio  of  the  applied  volts 
across  a  circuit  to  the  reactive  volts  induced  at  normal  full  load. 
If  the  circuit  pressure  and  the  reactive  voltage  are  known  for 
a  given  current  it  is  easy  to  calculate  the  voltage  usefully  em- 
ployed in  forcing  current  through  the  circuit  resistance  to  pro- 


Voltage  Drop"     2 
Power  fsctor--38 


%  Voltage  Drop-8~3 
Power  factor -3/7 

& 


WO  Volts 
Normal  Fu/l  Load 


100  Volts 
100%  Overload 


FIG.  73. 


duce  heat.  Since  the  circuit  pressure  is  the  vectorial  sum  of 
the  resistance  and  reactive  voltages  which  are  90°  out  of  phase, 
the  circuit  pressure  may  be  represented  graphically  by  the  hypo- 
tenuse of  a  right-angled  triangle,  the  other  sides  of  which  repre- 
sent the  resistance  and  reactive  volts.  The  resistance  volts  may 
then  be  calculated  as  the  square  root  of  the  difference  between 
the  squares  of  the  circuit  volts  and  reactive  volts. 


7.  Voltage  Drop -83 
Power  factor --3 17 


%  Voltage  Drop -40 
Power  factor  *  •  6 


100  Volts 
Normal  full  Load 


100  Volts 
/007*  Overload 


FIG.  74. 


The  diagrams  shown  in  Figs.  73  and  74  show  how  the  re- 
sistance volts  are  reduced  in  the  case  of  two  circuits  by  an  in- 
crease in  current  from  normal  full  load  to  100  per  cent,  overload  ; 
the  reactance  voltage  is  in  one  case  20  volts  and  in  the  other  case 
40  volts  at  normal  full  load  current,  the  constant  circuit  voltage 
being  100  for  both.  These  diagrams  show  that  the  voltage 
drop  increases  in  far  greater  proportion  than  either  the  current 
or  the  reactive  volts,  and  are  true  provided  that  the  magnetic 
circuits  are  such  as  to  allow  the  number  of  magnetic  lines  of 
force  to  increase  in  proportion  to  the  current. 

The  useful  or  power-developing  current  in  a  self-inductive 


GENERATION    OF   SINGLE   AND   POLYPHASE    CURRENTS         85 

circuit  is  in  phase  with  the  resistance  volts,  so  that  the  angle 
between  the  sides  of  the  triangle  representing  the  resistance 
and  circuit  volts  is  the  angle  by  which  the  current  "lags"  the 
applied  or  circuit  voltage,  and  therefore  the  magnitude  of  the 
resistance  volts  divided  by  the  applied  volts  ;  in  other  words,  the 
cosine  of  this  angle  is  the  power  factor.  These  diagrams  also 
show  how  the  power  factor  falls  on  overloads  as  a  result  of  in- 
troducing reactance. 

If  the  fixed  voltage  across  a  circuit  that  includes  a  reactance 
coil  and  a  practically  non-inductive  resistance,  such  as  an  arc, 
is  known,  it  is  possible  to  design  the  reactance  coil  to  produce 
a  desired  reactive  voltage  for  a  given  current,  so  that  the  current 
fluctuation  can  be  kept  within  desired  limits  by  the  consequent 
reduction  of  the  volts  available  for  overcoming  the  non-inductive 
resistance. 

In  the  case  of  Snyder  furnace  installations  the  reactive  vol- 
tage is  made  equal  to  the  resistance  voltage  at  normal  full  load, 
so  that  if  the  circuit  pressure  is  100  volts  the  voltage  available 
for  forcing  current  through  the  arc  resistance  is  equal 

^55?  =  70-7  volts. 

With  this  ratio  of  reactive  to  resistance  volts  at  normal  full 
load  it  is  impossible  for  the  current  on  dead  short  circuit  to  ex- 
ceed 1'41  times  the  full  load  current,  but  this  benefit  can  onty 
be  gained  at  the  expense  of  a  low  power  factor  equal  to  *707. 
The  curve  shown  in  Fig.  67  is  characteristic  of  the  effects  of 
such  a  reactance. 

In  some  cases  the  reactance  coils  contain  no  iron,  the  mag- 
netic circuit  being  in  air  alone.  The  advantage  of  this  type  is 
that  the  magnetic  lines  of  force,  and  therefore  the  reactive  vol- 
tage, are  directly  proportional  to  the  current ;  the  length  of  wire 
used  in  each  coil  is,  however,  much  greater  than  for  iron  core 
choking  coils. 

Reactance  coils  are  used  essentially  to  restrict  current  fluc- 
tuation, and  are  therefore  unnecessary,  and  in  fact  undesirable, 
if  the  load  is  steady.  Various  arrangements  have  been  proposed 
in  which  reactance  coils  are  used  in  conjunction  with  change- 
voltage  transformer  switches,  so  that  they  are  only  included  in 
the  power  circuits  when  operating  at  a  high  voltage,  and  at  a 
time  when  the  load  is  fluctuating. 


CHAPTEK  V. 

AUTOMATIC  KEGULATOKS  AND  ACCESSORIES. 

IN  all  arc  and  arc-resistance  furnaces  the  current  flowing 
through  any  electrode  is  dependent,  apart  from  the  effects  of 
reactance,  upon  its  circuit  resistance,  which,  being  mainly  in 
the  arc  itself,  is  always  subject  to  either  rapid  or  slow  variation. 

The  non-inductive  resistance  of  an  indirect  arc  circuit  lies 
in  an  arc  gap,  the  length  of  which  can  only  vary  by  consumption 
of  the  electrodes  or  by  deliberate  adjustment.  The  resistance 
of  an  arc  of  given  length  is  also  dependent  upon  the  temperature 
and  nature  of  the  atmosphere  through  which  it  passes,  but  if 
these  remain  constant  the  resistance  of  the  arc  is  also  unaltered. 
For  this  reason,  after  striking  the  arc  of  an  indirect  arc 
furnace,  the  load  will  remain  fairly  steady,  and  adjustment  of 
the  electrodes  will  only  be  occasionally  necessary  to  compensate 
for  their  consumption  and  the  slowly  varying  temperature  and 
character  of  the  furnace  gases. 

The  case  of  the  direct  arc  is  very  different.  Neglecting  the 
minor  considerations  of  furnace  gases,  the  resistance  of  an  arc, 
and  therefore  the  current  flowing  through  it  at  a  given  arc 
voltage,  will  depend  upon  the  length  of  the  arc  gap  as  deter- 
mined by  the  position  of  the  electrodes,  one  of  which  is  con- 
stituted by  the  charge  being  melted  or  the  charge  after  fusion. 
During  the  melting-down  operation  the  length  of  the  arc  gap 
will  always  be  increasing,  so  long  as  the  metal  melted  can  flow 
away  from  the  carbon  electrode,  and  this  proceeds  until  ultimately 
a  bath  is  formed.  Therefore,  to  maintain  a  constant  load  it 
would  be  necessary,  theoretically,  to  advance  the  carbon  electrode 
continuously  so  as  to  maintain  always  an  arc  of  uniform  length. 
This  is  obviously  impossible  in  practice  as  so  many  other 
factors,  such  as  the  character  of  the  scrap,  the  size  of  elec- 
trodes, and  the  varying  resistance  to  the  passage  of  current 

(86; 


AUTOMATIC   REGULATORS   AND   ACCESSORIES  87 

offered  by  the  charge  itself,  influence  the  distribution  of  load, 
so  that  the  problem  becomes  not  merely  one  of  maintaining  a 
constant  arc  length,  but  rather  one  of  constantly  adjusting  the 
arc  length  to  compensate  for  a  variable  resistance  in  the  other 
parts  of  the  circuit. 

If  a  charge  consists  of  very  heavy  tightly  packed  scrap, 
then  the  only  part  of  the  arc  circuit  in  which  the  resistance  can 
vary  appreciably  is  in  the  arc  gap  itself,  and  this  will  remain 
practically  constant  so  long  as  the  metal  melted  does  not  flow 
away  and  so  lengthen  the  arc.  Such  conditions  of  melting 
cold  scrap  are,  however,  both  undesirable  and  seldom  possible 
in  practice,  so  that  during  the  greater  period  of  the  melting- 
down  stage  the  arc  lengths  require  frequent  adjustment,  and 
in  fact  to  an  extent  that  usually  renders  automatic  load  regula- 
tion desirable  in  all  but  small  capacity  furnaces. 

Electric  motors  are  used  on  all  large  modern  arc  furnaces 
for  operating  the  electrode  adjusting  gear,  and  the  various  types 
of  automatic  regulators  are  all  designed  to  control  these  motor 
circuits. 

The  principle  of  their  operation  consists  of  closing  the  motor 
circuits  in  such  a  way  that  the  direction  of  rotation  either  ad- 
vances or  withdraws  the  electrode  so  soon  as  the  K.  V.A.  load 
on  any  arc  circuit  slightly  falls  below  or  exceeds  a  certain  de- 
sired figure.  In  every  case  the  automatic  regulator  operates 
by  means  of  an  electro-magnetic  device,  which  is  actuated  by  a 
current  that  bears  a  direct  relationship  either  to  the  current 
flowing  through  each  arc  circuit  or  to  the  voltage  across  each 
arc  itself.  The  "load"  depends  both  on  the  current  flowing 
and  the  voltage  across  each  arc  circuit,  so  that  if  one  of  these 
factors  is  always  constant,  it  can  be  regulated  in  accordance 
with  the  variation  of  the  other. 

In  the  original  single-phase  Heroult  furnace,  in  which  two 
arcs  are  in  series,  it  is  evident  that  unless  the  circuit  voltage  is 
equally  divided  between  the  two  arcs  the  power  absorbed  in 
each  arc  will  not  be  equal ;  this,  of  course,  follows  as  a  result  of 
the  current  through  both  arcs  being  the  same  at  any  moment, 
since  they  are  in  series.  In  this  case,  therefore,  regulation 
must  necessarily  be  effected  according  to  the  voltage  across  each 
arc,  but  this  only  ensures  equal  distribution  of  load  without 


88  THE    ELECTRO-METALLURGY   OF    STEEL 

controlling  the  magnitude  of  the  load  which  depends  also  upon 
the  current  flowing.  Owing  to  the  difficulties  of  operating 
these  single-phase  furnaces  on  public  supply  mains,  it  is  usually 
necessary  to  install  a  special  generating  plant.  The  character- 
istics of  suitable  alternators,  as  described  in  Chapter  IV.,  are 
such  that  the  terminal  or  line  voltage  falls  as  the  current  rises 
and  vice  versa,  and  a  certain  current  output  corresponds  to 
a  fixed  terminal  voltage.  For  this  reason,  voltage  regulation 
is  alone  necessary  to  determine  the  total  load  and  its  equal 
division  between  the  two  arcs. 

If  static  transformers  are  used  in  place  of  these  generators, 
then  the  line  voltage  would  be  constant  and  the  current  mainly 
dependent  upon  the  arc  circuit  resistance.  Voltage  regulation 
of  the  arcs  would,  under  these  conditions,  only  control  the 
division  of  the  total  load  but  not  its  magnitude,  so  that  adjust- 
ment of  each  electrode  according  to  the  current  flowing  through 
the  circuit  is  equally  necessary.  Therefore,  in  the  case  of  any 
furnace  having  two  arcs  in  series  and  taking  power  off  a  con- 
stant voltage  supply,  it  is  necessary  to  regulate  the  electrodes 
according  to  both  the  current  flowing  through  the  circuit  and 
the  voltage  across  each  arc. 

Polyphase  furnaces  operated  by  static  transformers  may  be 
broadly  divided  into  two  classes  for  the  purpose  of  considering 
the  requirements  of  automatic  regulation : — 

I.  Furnaces  in  which  the  voltage  across  each  arc  and  the 
current  flowing  through  any  arc  circuit  are  entirely  independent 
of  the  operation  of  other  arcs. 

II.  Furnaces  in  which  both  the  voltage  across  each  arc  and 
the  current  flowing  through  any  arc  circuit  are  dependent  upon 
the  mutual  operation  of  other  arcs. 

To  the  first  class  belong  two-phase  two-arc  furnaces  of  the 
Electro-metals,  Booth  Hall  and  Eennerfelt  type,  when  in  their 
operation  the  resistance  between  each  arc  and  the  neutral  return 
conductor  is  negligible,  thus  rendering  each  arc  circuit  entirely 
independent  of  the  other.  In  the  case  of  conductive  hearth 
furnaces  of  the  Electro-metals  type,  this  condition  is  only 
fulfilled  when  the  bottom  is  at  a  high  temperature  and  has  very 
low  resistance.  Kegulation  of  load  under  such  conditions  is 
then  accomplished  solely  by  regulating  the  electrodes  to  carry  a 


AUTOMATIC   REGULATORS   AND   ACCESSORIES  89 

definite  current,  the  arc  voltage  remaining  fixed  within  narrow 
limits  and  only  dependent  upon  the  characteristics  of  the 
transformers. 

To  the  second  class  belong  three-arc  three-phase  furnaces  of 
the  Heroult  type.  Since  the  three  arcs  are  star  connected  the 
arc  voltages  are  equal  when  the  current  is  equal,  but  if  the  con- 
dition of  balance  is  upset  by  the  resistance  of  one  arc  suddenly 
being  diminished,  then  the  current  flowing  through  that  arc  will 
increase  and  at  the  same  time  cause  a  corresponding  but  smaller 
current  rise  through  the  other  arc  circuits  ;  the  voltage  across 
the  arc  of  reduced  resistance  will  also  fall,  and  in  this  case 
cause  the  voltage  across  the  other  arcs  to  rise.  This  rise  of 
voltage  is  inevitable  since  the  line  voltage  is  only  slightly 
variable,  and  accounts  for  the  constant  flickering  of  the  arc 
voltage  indicator  lamps  during  melting  operations.  [Regulation 
by  the  current  flowing  through  each  electrode  is  generally 
adopted,  but  to  prevent  an  overload  on  one  arc  from  causing 
displacement  of  the  other  electrodes,  which  may  themselves  be 
in  correct  position  relative  to  the  bath,  it  is  necessary  to  adjust 
the  regulators  to  operate  only  outside  certain  limits  of  current 
fluctuation ;  otherwise,  if  balance  were  once  upset,  all  three 
regulators  would  be  in  constant  oscillatory  operation  without 
ever  effecting  balance.  Further,  in  furnaces  of  this  class  it  is 
possible  for  an  electrode  to  touch  the  charge  or  penetrate  the 
bath  without  any  current  flowing  until  one  of  the  other  electrodes 
completes  the  circuit,  and  for  this  reason  automatic  regulation 
dependent  upon  current  flow  alone  will  actually  force  an 
electrode  downwards  until  the  circuit  is  completed  by  one  of 
the  other  electrodes.  To  overcome  this  objection  a  special  form 
of  regulator,  which  operates  in  accordance  with  the  voltage 
between  each  electrode  and  the  furnace  charge,  is  sometimes 
employed  in  conjunction  with  regulators  that  are  operated  ac- 
cording to  current  flow. 

To  improve  the  sensitiveness  of  automatic  regulators  it  has 
been  proposed  to  operate  them  by  two  electro-magnets  drawing 
current  from  different  sources,  the  current  in  one  case  being 
dependent  upon  the  current  flowing  through  each  electrode 
circuit,  and  in  the  other  case  upon  the  voltage  across  each  arc. 
In  this  case  the  regulation  of  individual  electrodes  would  be 


90  THE   ELECTEO-METALLUEGY   OF    STEEL 

more  independent  of  the  disturbing  effect  produced  by  a  varying 
current  through  others. 

Thury  Regulator. — This  regulator  consists  essentially  of 
three  distinct  parts  assembled  together  on  the  same  framework. 

(a)  An  electro-magnetic  device  actuated  by  a  current  which 
at  all  times  bears  a  definite  relation  either  to  the  current  flowing 
through  the  electrode  circuit  or  to  the  voltage  across  each  arc. 
It   is  also  designed  so  that  a  moving  element  of   the  electro- 
magnet  remains   in    a   position  of   equilibrium   for  a  definite 
energising  current. 

(b)  A  purely  mechanical   device,  which  is  only  caused    to 
operate  when    the  equilibrium  of   the  moving  element  of  the 
electro-magnet  is  disturbed  by  variation  of  the  energising  current. 

(c)  Carbon  terminals  connected  to  the  motor  circuits  and 
mounted  on  the  regulator,  enabling  either  one  of  two  possible 
circuits  to  be  closed  so  that  the  motor  drive  may  be  in  either 
a  forward  or  backward  direction.     The  proper  connections  are 
made  to  the  carbon  terminals  by  the  mechanically  driven  part 
of  the  regulator,  which  also  carries  carbon  terminals  connected 
to  the  motor  circuits. 

The  above  brief  description  will  enable  the  operation  of  the 
regulator  shown  in  Fig.  75  to  be  more  easily  understood. 

The  electro-magnet  F  is  connected  to  a  circuit  in  which  the 
current  flowing  always  bears  a  definite  relationship  to  the  current 
passing  through  the  electrode  circuit  or  to  the  voltage  across  the 
arc.  The  current  flowing  through  the  coils  of  the  electro-magnet 
causes  the  repulsion  of  a  rectangular  copper  ring  B,  which  is 
free  to  move  between  the  pole-pieces  of  the  electro-magnet  F. 
This  copper  ring  B  is  attached  to  a  lightly  pivoted  arm  E,  which 
carries  at  one  end  a  double  knife-edged  striker  C  ;  to  the  other 
end  are  fixed  a  light  tension  spring  A  and  a  freely  suspended 
rod,  which  carries  two  small  collars  and  terminates  in  an  adjust- 
able piston  valve  working  in  a  dashpot  N.  The  weight  of  the 
arm  E  and  the  parts  attached  are  so  balanced  that  the  force  of 
repulsion  due  to  a  particular  current  will  overcome  the  tension 
of  the  spring  and  cause  the  arm  to  remain  in  equilibrium  in  a 
horizontal  position.  Movement  of  the  pivoted  arm  is  extremely 
delicate  and  responds  to  small  variations  of  current.  The  dash- 
pot  N  is  introduced  to  prevent  the  arm  responding  to  momen- 


E- 


U 


FIG.  75. 


[To  face  p.  90. 


AUTOMATIC   REGULATORS   AND   ACCESSORIES  91 

tary  fluctuation  of  current,  and  to  cause  a  slow  steady  movement 
in  either  an  upward  or  downward  direction  instead  of  sudden 
jerks.  A  vertical  member  mounted  on  the  central  spindle  is 
continuously  oscillated  to  and  fro  by  the  simple  crank  drive 
shown,  and  the  head  of  this  member  carries  two  tappets  K,  K' 
which  engage  the  pawls  I  and  I'.  A  notched  wheel  H  is  also 
mounted  on  the  same  central  spindle  and  actuates  a  vertically 
hanging  arm  to  which  are  connected  two  insulated  carbon 
terminal  blocks  Z,  Z.  The  front  and  lower  frame  of  the  regulator 
carries  two  pairs  of  insulated  carbon  terminal  blocks,  which, 
together  with  the  pair  on  the  vertical  arm,  are  suitably  connected 
to  the  motor  circuits. 

When  the  current  flowing  through  the  electro-magnet  is  just 
sufficient  to  balance  the  pivoted  arm  E  in  a  horizontal  position, 
then  the  strikers  C  will  just  miss  the  tappets  K,  K'  every  time 
the  latter  are  oscillated  towards  and  beyond  each  knife  edge 
respectively,  so  that  no  movement  of  the  notched  wheel  occurs. 
If  the  current  should  increase  sufficiently  to  disturb  the  equili- 
brium of  the  arm  E,  the  striker  C  will  be  depressed  and 
engage  the  tappet  K'  ;  at  that  moment  the  pawl  I'  will  have 
passed  the  notch  during  its  oscillation  towards  the  left,  and 
being  released  by  the  tappet  K'  will  drop  into  the  notch  on  its 
return  movement  to  the  right.  The  notched  wheel  H  will  then 
be  advanced  in  a  clockwise  direction,  and  by  carrying  the  vertical 
arm  with  it,  will  close  the  various  circuits  to  cause  rotation  of 
the  motor  in  a  direction  to  reduce  the  current  flowing  through 
the  electro-magnet.  If  this  current  is  in  relation  to  the  current 
flowing  through  the  electrode  circuit,  then  the  motor  will  adjust 
the  electrode  to  lengthen  the  arc,  but  if  in  relation  to  arc  vol- 
tage, the  motor  will  reduce  the  length  of  the  arc.  The  time 
during  which  the  contacts  are  held  may  be  adjusted  by  moving 
the  carbon  terminals  X  either  towards  or  away  from  the  central 
vertical  arm  and  terminal  blocks.  When  the  movement  of  the 
pawl  I'  towards  the  right  has  been  completed,  the  direction  of 
movement  is  reversed  and  the  notched  wheel  will  be  restored  to 
its  central  position ;  the  pawl  I'  at  the  same  time  will  be  auto- 
matically lifted  out  of  the  notch  and  once  more  engaged  and 
held  by  the  tappet  K',  unless  the  striker  C  is  still  depressed,  when 
repetition  of  the  above  movements  again  proceeds. 


92  THE    ELECTKO-METALLUKGY   OF    STEEL 

The  adjustment  of  electrodes  is,  therefore,  effected  by  a  step- 
by-step  movement,  which  is  intended  to  prevent  the  regulation 
of  load  being  carried  too  far  in  the  reverse  direction,  which  would 
then  cause  continual  regulation  or  hunting.  To  make  this  step- 
by-step  movement  more  pronounced  the  pivoted  arm  E,  after 
being  raised  or  depressed  to  cause  movement  of  the  notched 
wheel,  is  returned  to  its  mid-position  of  equilibrium,  irrespective 
of  the  other  balancing  forces  acting  upon  it.  This  is  effected  by 
means  of  a  pivoted  lever  M  carrying  at  one  end  a  pin  which 
projects  forward  between  two  studs  fixed  to  the  periphery  of  the 
notched  wheel.  The  other  end  of  the  lever  M  terminates  in  a 
fork,  which  on  its  upward  or  downward  stroke  pushes  the  collars 
n  or  n  respectively  to  a  position  that  brings  the  pivoted  lever 
E  back  to  its  mid-position  of  equilibrium.  This  latter  move- 
ment of  the  lever  M  completes  the  entire  cycle  of  mechanical 
movements  resulting  from  the  release  of  either  pawl  I  or  T  by 
means  of  the  striker  C  and  tappets  K,  K'. 

If  during  one  movement  of  the  notched  wheel  H  the  current 
through  the  electro-magnet  is  not  restored  to  normal,  then  the 
lever  E  will  be  again  free  to  move  and  produce  a  further  cycle 
of  movements.  By  means  of  the  return  action  above-mentioned 
and  the  dashpot  effect  which  is  adjustable,  the  time  interval 
between  each  successive  step-by-step  movement  is  made  subject 
to  control.  The  small  set  screws  T  are  adjusted  so  as  to  limit 
the  movement  of  arm  E,  and  so  only  allow  the  striker  C  to  rise 
or  fall  no  further  than  is  just  necessary  to  engage  a  small  V- 
shaped  groove  cut  in  the  face  of  each  tappet  K,  K'.  The  collars 
n  and  n  should  also  be  fixed  to  the  dashpot  rod  so  that  the 
fork  on  the  lever  M  just  brings  the  striker  C  back  to  mid  posi- 
tion at  the  limit  of  its  stroke. 

A  furnace  regulator  must  be  capable  of  maintaining  a  fairly 
constant  load  over  a  wide  range,  which  is  generally  from  full 
load  to  quarter  load,  so  that  the  relation  between  the  current 
flowing  through  the  electro-magnet  device  and  the  current  flow- 
ing through  the  electrode  circuit  must  be  capable  of  definite 
variation. 

For  purposes  of  current  control  a  current  transformer  of 
fixed  ratio  is  placed  in  the  electrode  circuit  and  connected  in 
series  with  the  electro-magnet  windings.  The  current  flowing 


AUTOMATIC   REGULATORS   AND   ACCESSORIES  93 

through  the  electro-magnet  will  then  be  a  definite  fraction  of  the 
current  flowing  through  the  electrode  circuit,  and  the  regulators 
would  only  be  capable  of  maintaining  one  fixed  current  value. 
To  increase  the  range  of  regulation  it  is  necessary  to  alter  the 
ratio  of  the  current  flowing  through  the  electro-magnet  to  the 
current  in  the  electrode  circuit,  and  this  is  done  by  placing  an 
adjustable  resistance  in  parallel  with  the  electro-magnet  wind- 
ings. Thus,  by  varying  this  shunt  resistance,  the  current 
generated  in  the  current  transformer  will  be  divided  between 
the  electro-magnet  and  shunt  resistance  circuits  according  to 
their  relative  resistances. 

Supposing  5  amperes,  flowing  through  the  electro-magnet, 
are  just  sufficient  to  maintain  equilibrium,  then  if  this  amperage 
were  reduced  by  shunting  a  portion  through  a  resistance, 
equilibrium  would  be  upset  and  the  regulator  would  operate 
until  the  increased  load  again  raised  the  current  through  the 
electro-magnet  windings  to  5  amperes.  In  this  way,  by  varying 
the  resistance  in  parallel  with  the  electro-magnet  circuit,  the 
regulator  may  be  set  to  operate  at  different  loads.  A  diagram 
of  connections  for  a  Thury  regulator  controlling  an  electrode 
adjusting  motor  is  shown  in  Fig.  76,  which  indicates  how  the 
circuit  connections  made  in  a  tramway  type  controller  are  com- 
pleted on  the  regulator  by  movement  of  the  two  centre  terminal 
carbons  ;  the  position  of  the  current  transformer  relative  to  the 
electrode  circuit  is  also  shown. 

The  function  of  a  voltage  control  regulator  is  usually  to 
maintain  a  definite  fixed  voltage  across  an  arc,  and  in  this 
respect  is  only  used  to  divide  a  load  equally  between  two  or  more 
arcs  without  directly  controlling  the  magnitude  of  the  current 
flowing ;  this  applies  to  the  regulation  of  arcs  in  series,  where 
the  current  flowing  through  the  circuit  is  separately  controlled 
by  a  current  control  regulator.  The  Thury  regulator  is  a 
delicate  piece  of  mechanism  and  requires  very  careful  adjust- 
ment and  attention  ;  otherwise  correct  regulation  is  not  obtained. 

Watford  Regulators. — The  Thury  regulator  as  used  on 
two-phase  and  three-phase  furnaces  is  essentially  a  current 
control  regulator,  and  has  the  disadvantage  that  one  electrode 
may  be  forced  on  to  a  charge  until  its  circuit  has  .been  com- 
pleted to  permit  of  the  normal  current  flowing.  The  Watford 


94 


THE   ELECTED -METALLURGY   OF    STEEL 


superimposed  voltage  regulator,  which  overcomes  this  objection, 
is  purely  a  voltage  control  instrument  and  operates  in  conjunc- 
tion with  the  Thury  or  other  current  control  type  of  regulator. 
This  voltage  regulator  when  so  used  is  inoperative  until  the 
voltage  between  an  electrode  and  the  furnace  charge  has  fallen 

Direct  Current  Auxiliaries  Supply 


Automatic  Regulator      ^trode  Motor    \ 


Potentfomefsi 
Resistance 


vk 

Regulator     j 
Motor 


L  oad  Adjusting     \  £"  ;vf &  '\ 
Resistance 


Shunt 


N°l  Regulator  F!^ 
Equipment 


Current  Transformer 
FIG.  76. 

below  a  minimum  figure,  which  might  result  either  from  a 
short-circuited  overload,  or  from  contact  being  made  with  the 
charge  without  any  flow  of  current,  as  in  the  case  above  ex- 
plained. A  sudden  drop  in  the  arc  voltage  causes  a  switch  to 
open  which  disconnects  the  motor  from  the  current  control 
regulator,  and  at  the  same  time  closes  another  switch  which 


AUTOMATIC   REGULATORS   AND   ACCESSORIES  95 

re-connects  the  motor  circuits  so  as  to  produce  rotation  in  a 
direction  to  withdraw  the  electrode  from  the  charge  until  the 
arc  voltage  is  re-established ;  the  withdrawal  is  continuous  and 
therefore  more  rapid  than  with  the  step-by-step  type,  so  that 
short-circuit  overloads  are  rapidly  corrected.  When  the  arc 
voltage  has  been  restored,  the  current  control  regulator  again 
becomes  operative,  but  only  after  the  motor  has  been  pulled  up, 
which  prevents  "  hunting". 

The  Watford  regulator,  as  distinct  from  the  superimposed 
voltage  control  instrument,  performs  the  dual  function  of 
regulating  according  to  the  current  flowing  through  each 
electrode,  as  in  the  Thury  type,  and  to  a  voltage  drop  below  a 
fixed  minimum  figure,  this  latter  operation  being  identical  with 
the  superimposed  voltage  control  described  above. 

The  electro-magnetic  device,  which  for  current  control  pur- 
poses is  connected  to  the  current  transformer  in  the  electrode 
circuit,  consists  of  a  pivoted  armature  capable  of  rotation 
between  the  two  poles  of  an  electro-magnet.  Two  copper  bands 
are  recessed  in  this  armature  and  set  up  forces  tending  to 
rotate  it  against  an  opposing  mechanical  force.  The  armature 
assumes  a  definite  position  of  equilibrium  according  to  the 
current  flowing  through  the  electro-magnet  coils.  Variation 
of  current  causes  a  slight  rotary  movement  which  closes  a  light 
relay  circuit ;  this  relay  then  closes  another  circuit  operating  a 
clapper  switch,  which  again  closes  the  motor  circuit.  Accord- 
ing to  the  direction  of  rotation  of  the  armature,  either  one  of 
two  clapper  switches  is  thus  set  in  operation,  so  that  the  motor 
is  capable  of  rotation  in  either  direction.  By  means  of  other 
switches  and  solenoid  devices  the  operation  of  the  clapper- 
switch  is  made  intermittent,  so  that  adjustment  of  the  electrode 
resembles  the  step-by-step  movement  of  the  Thury  type  which 
prevents  "  hunting  ".  The  voltage  regulator  is  similar  in  action 
to  that  previously  described,  and  operates  in  conjunction  with 
the  current-regulating  portion  of  the  apparatus. 

General  Electric  Company,  U.S.A.,  Regulator. — This  regula- 
tor operates  on  somewhat  similar  principles  to  the  Watford  in- 
strument, embodying  current  regulation  and  no-voltage  control 
devices. 

The  electrode  motors  are  operated  by  contactor  switches, 


96  THE    ELECTKO-METALLTJKGY    OF    STEEL 

which  are  themselves  controlled  by  a  contact-making  ammeter 
device  or  electro-magnet.  This  latter  instrument  is  con- 
nected to  a  current  transformer  and  assumes  a  position  of 
equilibrium  when  energised  by  a  definite  current.  Current 
variation  either  above  or  below  this  figure  causes  displacement 
of  the  ammeter's  moving  element,  which  closes  one  of  two  con- 
tactor switch  circuits  so  as  to  operate  the  motor  either  to 
reduce  or  increase  the  current  flow  through  the  electrode 
circuit. 

Auxiliary  contactor  switches  interlocked  with  the  motor 
circuit  contactors  are  also  provided,  and  are  closed  by  the 
counter  E.M.F.  generated  by  the  motor  as  soon  as  the  latter's 
main  circuits  are  opened  after  the  load  readjustment  has  been 
completed.  In  this  way  the  motors  are  rapidly  brought  to  rest 
by  a  dynamic  braking  effect.  Low  voltage  relays  are  provided 
to  render  the  entire  apparatus  inoperative  the  moment  the 
voltage  between  any  pair  of  the  furnace  conductor  bars  .falls 
away  owing  to  failure  of  the  power  supply ;  this  prevents 
damage  being  done  by  a  continuous  movement  of  the  electrodes 
towards  a  charge  in  the  endeavour  to  establish  a  normal 
current.  These  protective  relay  magnets  do  not,  however, 
respond  to  a  short  circuit  between  any  one  electrode  and  the 
furnace  charge,  so  that,  unless  such  short  circuit  is  accompanied 
by  a  current  either  equal  to,  or  exceeding  the  normal  equilibrium 
value,  the  motor  will  continue  to  force  the  electrode  in  a  direc- 
tion to  re-establish  normal  current.  The  regulator  may  be  set 
to  maintain  fixed  current  values  over  a  certain  range,  this  being 
done  by  cutting  out  parts  of  the  ammeter  windings,  and  so 
varying  its  magnetic  force  for  different  energising  currents. 

Furnace  Control  Instruments  :  Ammeters. — Ammeters  are 
used  to  control  the  load  of  furnaces  working  on  constant 
pressure  circuits,  and  to  divide  it  between  the  various  elec- 
trode circuits  to  secure  balance.  A  separate  ammeter  is 
operated  by  a  current  transformer  placed  in  each  electrode  cir- 
cuit and  is  calibrated  to  indicate  the  main  current  flowing. 
These  instruments  are  equally  essential  for  both  hand  and 
automatic  control,  being  necessary  in  the  latter  case  for  setting 
the  load-adjusting  resistance  so  that  each  regulator  maintains 
the  desired  current.  They  are  always  mounted  in  front  of  or 


AUTOMATIC   REGULATORS    AND   ACCESSORIES  97^ 

close  to  the  electrode  regulating  motor  control  gear,  which  may 
be  either  a  tramway  type  controller  or  a  push  button  clapper 
switch  starter.  In  many  furnace  installations  all  the  instru- 
ments and  switch  gear  are  mounted  on  one  board  placed  in  front 
of  the  motor  controllers,  and  in  some  convenient  position  away 
from  the  furnace  ;  in  other  cases  the  ammeters  are  mounted 
on  a  small  panel  attached  to  the  back  of  the  furnace  with  the 
controllers  fixed  in  a  position  whence  the  instruments  are 
plainly  visible. 

Current  Transformers. — The  same  current  transformers  are 
generally  used  for  operating  the  regulator  electro-magnet 
together  with  all  control  instruments,  which  may  include  an 
ammeter,  indicating  wattmeter,  integrating  watt-hour  meter  and 
recording  wattmeter.  These  instruments  are  all  placed  in 
series  in  the  current  transformer  circuit,  the  load  adjusting  re- 
sistance alone  being  shunted  across  the  regulator  electro- 
magnet windings. 

Current  transformers  are  designed  with  a  fixed  current 
ratio,  so  that  the  current  flowing  through  the  instrument  circuit 
is  always  a  definite  fraction  of  the  current  flowing  through  the 
electrode  circuit.  This  is  true  in  general  practice,  since  the  re- 
sistance of  the  instrument  circuit  is  kept  well  below  a  figure 
which  might  otherwise  alter  the  current  transformer  ratio,  and 
so  vitiate  the  instrument  readings.  Spark  gaps  are  usually 
provided  which  short  circuit  the  secondary  windings  if  their 
circuit  is  broken  anywhere  ;  this  is  done  as  a  measure  of  safety 
to  overcome  all  risk  from  the  high  voltage  induced  across  the 
secondary  windings  of  the  current  transformer  should  its  circuit 
be  broken.  The  spark  gap  simply  consists  of  a  small  piece  of 
tissue  paper  placed  between  two  short-circuiting  contact  studs ; 
should  the  voltage  at  any  time  rise  to  dangerous  proportions  the 
paper  is  immediately  punctured  and  the  secondary  windings 
become  short  circuited.  Puncturing  of  the  tissue  paper,  which 
may  at  times  be  accidental,  is  often  the  cause  of  ammeters 
failing  to  indicate,  so  that  it  is  always  wise  to  examine  the 
spark  gaps  in  the  event  of  instrument  breakdown. 

When  current  transformers  are  required  to  operate  the 
electro-magnets  of  most  automatic  regulators,  the  ratio  of  the 
primary  to  secondary  currents  is  so  chosen  that  the  secondaiy 

7 


98  THE   ELECTEO-METALLUKGY   OF    STEEL 

current  at  normal  full  load  is  at  least  three  times  the  current 
required  to  maintain  the  electro-magnetic  device  in  normal 
equilibrium ;  under  these  conditions  the  regulator  will  be  able 
to  maintain  a  load  equivalent  to  one-third  full  load  current  or 
less.  This  is  important,  as  it  is  often  necessary  to  hold  a  bath 
of  steel  at  a  constant  temperature,  which  sometimes  can  only 
be  done  at  a  load  equivalent  to  one-quarter  or  one-third  of 
normal  full  load  current. 

Indicating  Wattmeter. — This  is  a  most  useful  instrument, 
if  intelligently  used,  since  it  indicates  at  a  glance  the  actual 
amount  of  power  being  taken  at  any  moment.  The  furnace 
load  in  K.V.A.  can,  of  course,  be  calculated  when  the  ammeter 
readings  are  all  steady  and  similar  and  the  line  voltage  known. 
Provided  the  line  voltage  is  never  changed  the  furnace  load 
might  be  controlled  by  use  of  the  ammeters  alone,  but  even 
then  the  load  adjustment  is  only  approximate  and  far  more 
difficult  under  a  fluctuating  load  than  with  an  indicating  watt- 
meter, which  indicates  the  sum  total  effect  of  possibly  two  or 
more  unbalanced  and  fluctuating  electrode  circuits.  Watt- 
meters are  usually  well  damped  and  their  value  for  power  in- 
dicating and  control  purposes  is  just  as  great  as  the  other 
instruments,  whose  function  is  more  especially  for  the  equal 
division  and  balancing  of  the  load.  It  is  advisable  that  furnace 
operators  should  become  accustomed  to  maintain  a  desired  load 
by  use  of  the  indicating  wattmeter,  and  at  the  same  time  to 
balance  that  load  by  adjusting  the  electrodes  so  that  the  am- 
meter readings  are  approximately  similar  or  within  the  same 
range  of  oscillation,  if  the  load  is  fluctuating.  An  indicating 
wattmeter  is  usually  operated  entirely  from  the  secondary 
circuits  of  the  furnace  transformers,  the  current  coils  being  in 
the  current  transformer  circuits  and  the  potential  coils  connected 
across  the  line  conductors. 

Graphic  Recording  Wattmeter. — This  instrument  is  most 
useful,  as  it  gives  on  a  paper  chart  a  complete  history  of  the 
electrical  operation  of  a  furnace  from  the  beginning  to  the  end 
of  a  heat ;  the  varying  average  magnitude  of  the  power  in 
kilowatts,  the  comparative  degree  of  fluctuation,  and  the  number 
and  length  of  every  stoppage  are  all  plainly  recorded.  This 
information  enables  anyone  to  judge  whether  the  furnace 


AUTOMATIC   EEGULATOES   AND   ACCESSOEIES  99 

manipulation  throughout  was  satisfactory  in  point  of  view  of 
economy  of  production  and  maximum  output,  each  of  which 
naturally  follows  from  operation  at  a  highest  possible  load 
factor. 

The  causes  contributing  to  a  low  load  factor  will  be  shown  as 
actual  stoppages  or  low  average  loads  at  different  periods  due 
either  to  excessive  fluctuation,  or  simply  to  poor  load  control ; 
in  each  case  the  chart  will  indicate  the  cause,  and  so  enable 
it  to  be  corrected.  A  recording  wattmeter  is  not  generally 
mounted  near  the  furnace,  as  it  is  only  intended  for  supervision 
purposes  and  not  as  a  guide  to  the  furnace  operators,  the  in- 
dicating wattmeter  being  quite  sufficient  for  this  latter  purpose. 
The  connections  are  made  similarly  to  the  indicating  wattmeter 
on  the  low  tension  power  transformer  circuits. 

Integrating  Watt- hour  Meter. — This  instrument  is  essen- 
tial as,  apart  from  measuring  the  units  used  for  payment  pur- 
poses, it  enables  the  power  consumption  per  ton  of  steel  for 
melting  and  for  preheating  the  furnace  to  be  calculated.  It 
therefore  provides  information  of  the  greatest  importance  for 
purposes  of  checking  production  costs.  Here  again,  the  instru- 
ment is  best  mounted  in  an  office  or  sub-station  where  it  is 
unexposed  to  heat  and  dust. 

The  connections  are  usually  made  in  the  same  way  as  for 
the  indicating  and  recording  wattmeters ;  the  readings  in  this 
case,  although  not  strictly  accurate,  are  quite  near  enough  for 
furnace  supervision  purposes,  and  may  only  slightly  differ 
from  the  readings  of  a  power  company's  instrument  operating 
off  the  high  tension  circuits  of  the  furnace  transformers. 

Furnace  Switchgears. — Furnaces,  drawing  power  from  a 
high  tension  supply  through  the  medium  of  static  transformers, 
are  connected  to  and  disconnected  from  the  source  of  supply  by 
an  oil  switch  placed  in  the  high  tension  circuits.  The  low 
tension  or  secondary  terminals  of  the  transformers  are  severally 
connected  direct  to  the  furnace  bus  bars,  although  in  certain 
cases  heavy  copper  knife  switches  are  interposed  either  for  the 
purpose  of  isolating  the  furnace  or  for  altering  the  low  tension 
cable  connections  from  the  transformer. 

Oil  switches  for  furnace  installations  should  be  of  very  strong 
construction,  with  the  oil  tanks  built  of  either  steel  plate  or  cast 


100  THE    ELECTROMETALLURGY    OF    STEEL 

steel.  The  copper  contacts  are  best  made  of  solid  copper 
instead  of  being  built  up  from  copper  strips.  Switches  are 
always  fitted  with  automatic  tripping  devices,  which  cause  the 
switch  to  open  in  the  e.vent  of  dangerous  overloads. 

Hand  controlled  switches  are  closed  by  a  handle,  which 
operates  through  a  system  of  levers  and  compresses  powerful 
springs  while  making  the  necessary  contacts  in  oil.  The  switch 
is  held  in  a  closed  position  against  the  springs  by  a  small  catch 
which  prevents  movement  of  the  levers ;  immediately  the  catch 
is  displaced  by  hand  or  other  means,  the  force  of  the  com- 
pressed springs  is  sufficient  to  open  the  circuits  with  great 
rapidity.  The  catch  is  so  arranged  that  it  can  be  released  by 
hand,  or  automatically  by  means  of  a  loose  iron  plunger  enclosed 
in  a  solenoid  and  carrying  a  striking  rod.  Should  the  current 
flowing  through  any  one  of  the  main  power  circuits  exceed  a 
safe  value,  the  solenoid  becomes  magnetised  and  attracts  the 
plunger.  These  tripping  coils  are  connected  to  a  group  of 
current  transformers,  so  that  the  pull  on  each  plunger  is  pro- 
portional to  the  current  flowing  through  the  main  circuit ;  to 
render  them,  however,  insensible  to  momentary  current  over- 
loads a  fuse  is  connected  in  parallel  with  each  solenoid  coil, 
and  by  its  low  resistance  carries  practically  the  full  current  in 
the  current  transformer  circuit.  In  this  way  the  action  of  the 
solenoid  is  prevented  until  the  fuse  melts,  as  the  result  of  either 
a  very  heavy  instantaneous  overload  or  of  a  more  prolonged 
overload  of  less  intensity.  A  time  element  is  thus  introduced 
which  prevents  the  otherwise  constant  tripping  due  to  momentary 
overloads.  This  method  of  automatic  tripping  is  very  simple, 
but  is  objectionable  owing  to  the  constant  renewal  of  fuses 
necessary,  and  to  the  fact  that  the  tripping  gear  and  fuses  are 
usually  in  an  exposed  position  near  the  furnace  and  liable  to 
misuse. 

Automatic  tripping  is  also  effected  by  overload  time  limit 
relays,  which  on  heavy  instantaneous  overloads  or  sustained 
overloads  of  less  intensity  close  an  auxiliary  circuit  through  the 
tripping  coils.  The  main  feature  of  certain  types  of  these  relays 
lies  in  their  being  self-setting  and  easily  adjusted  to  trip  over 
a  wide  range  of  instantaneous  overloads,  which  correspond  to 
smaller  overloads  over  certain  time  intervals.  This  instrument 


AUTOMATIC  BEGULATOKS  AND  kcic^SSOSlES        '    M01 


may  be  mounted  in  the  sub-station,  ai}d  scanty  peri&rts 
operation  of  the  switch  from  the  furnace  shop. 

The  oil  in  furnace  switches  should  be  examined  from  time 
to  time,  as  it  is  likely  to  become  carbonised  and  sludged  if  the 
switch  is  carelessly  used  for  breaking  and  making  circuit  under 
load. 


CHAPTEK  VI. 

POWEK  CONSUMPTION  COST  AND  CONTRIBUTORY  FACTORS. 

DURING  the  earlier  years  of  the  electric  steel  industry,  at  a  time 
when  the  full  value  and  scope  of  the  electric  furnace  had  not 
been  generally  recognised,  the  cost  of  power  for  melting  and 
refining  was  regarded  as  the  predominant  factor  of  economic 
success  or  failure.  At  that  time,  also,  the  electrical  efficiency  of 
single-phase  furnace  installations  was  poor,  and  as  the  field  of 
utility  was  then  very  limited  it  is  not  surprising  that  all  those 
other  economic  factors,  which  are  now  realised  to  be  of  equal 
importance,  were  subordinated  to  the  prime  consideration  of 
power  cost.  It  was  also  by  no  means  generally  accepted  that 
electric  steel  was  comparable  in  quality  to  crucible  steel,  and 
when  sold  in  the  form  of  billets  or  bars  it  was  often  placed  in 
the  same  category  as  Swedish  Bessemer  steel.  Under  these 
circumstances  the  power  consumption  cost  amounted  to  roughly 
one-quarter  or  one-third  of  the  selling  price,  when  steam  raised 
power  was  used. 

The  same  vital  importance  can  no  longer  be  attached  to  the 
question  of  power  cost  in  the  case  of  high-class  carbon  and 
alloy  steel  ingots,  which  now  command  a  market  value  con- 
siderably more  than  four  times  the  power  consumption  cost, 
notwithstanding  the  increased  cost  per  unit  of  steam-generated 
electricity.  Power  consumption  may  now  be  considered  of  only 
equal  importance  with  other  items  of  manufacturing  costs,  but 
should  receive  all  possible  consideration  from  both  adminis- 
trative and  technical  standpoints  with  a  view  to  reaching  a 
maximum  economy. 

Power  Contracts. — Steam  generated  electric  power  is  gener- 
ally purchased  under  either  the  "meter  rate,"  "  flat  rate,"  or 
''maximum  demand  rate"  systems.  In  each  case  the  power 

(102) 


POWER   CONSUMPTION    COST   AND   CONTRIBUTORY   FACTORS      103 

company's  charges  are  based  upon  the  "  Cost  of  Service  "  theory 
of  rate-making,  which  is  so  regulated  as  to  include  a  fair  return 
upon  invested  capital. 

Whenever  the  supply  of  power  in  bulk  is  contracted  for, 
plant  capable  of  supplying  that  amount  must  always  be  avail- 
able to  meet  the  demand,  so  that,  whether  the  consumer  is 
taking  power  or  not,  the  power  company  is  shouldered  with  a 
fixed  burden  of  depreciation,  administrative  expenses,  and 
interest  on  capital.  On  this  account,  under  the  different  systems 
of  rate-making,  the  consumer  is  required  to  make  certain  pay- 
ments which  serve  as  a  guarantee  to  the  power  company  against 
all  such  stand-by  losses.  Power  contracts,  for  this  reason,  are 
usually  based  upon  a  combination  of  either  flat  and  meter  rates, 
or  of  maximum  demand  and  meter  rates. 

Meter  Rate  System. — In  most  contracts  for  the  supply  of 
electric  light  service,  the  number  of  units  consumed  are  metered 
and  charged  for  at  a  fixed  rate  per  unit.  In  such  cases  the 
power  supply  company  is  well  acquainted  with  the  maximum 
bulk  and  nature  of  the  load,  or,  in  other  words,  the  approximate 
number  of  units  used  per  annum  and  the  load  factor.  Under 
these  circumstances  the  meter  rate  payment  can  be  regulated 
to  cover  all  estimated  stand-by  losses. 

There  are  various  modifications  of  this  system  which  either 
impose  an  extra  charge  or  allow  a  rebate,  according  to  whether 
the  load  is  taken  at  times  when  the  central  station  generating 
plant  is  fully  loaded  or  partly  idle.  In  the  case  of  lighting 
services  the  maximum  call  for  power  occurs  during  the  hours  of 
darkness,  during  part  of  which  the  central  station  load  is  at  its 
"peak". 

Flat  Rate  System. — According  to  this  system  no  meter  is 
used,  the  charge  for  electric  service  being  based  upon  the  power 
consuming  capacity  of  the  consumer's  installation,  or  upon  an 
agreed  fixed  sum  for  each  consumer.  Such  charge  would 
naturally  be  based  upon  the  possibility  of  overloads,  and  upon 
the  average  load  factor  likely  to  be  attained  on  the  consumer's 
installation,  which  is  a  measure  of  the  total  estimated  number  of 
units  consumed  per  annum. 

The  generating  cost  of  steam-raised  electric  power  is  con- 
siderable, and  requires  the  insertion  of  a  special  Coal  Clause  in 


104  THE   ELECTEO-METALLUEGY.    OF   STEEL 

every  power  contract.  In  the  case  of  hydro-electric  power, 
generating  cost  is  practically  negligible,  so  that,  if  the  power 
charge  is  based  upon  a  consumer's  fixed  plant  capacity,  it  is 
immaterial  whether  the  power  is  delivered  intermittently  or 
under  a  high  load  factor. 

The  flat  rate  system  alone  cannot  always  be  satisfactorily 
applied  in  the  case  of  steam-raised  power,  and  is  therefore 
usually  used  in  conjunction  with  the  meter  rate.  The  latter 
covers  the  central  station  generating  costs  and  profit,  whereas 
the  former  is  intended  to  cover  the  fixed  charges  on  the  plant, 
which  must  be  at  all  times  available  for  instant  use. 

Furthermore,  when  basing  the  flat  rate  charge  upon  the 
rated  plant  capacity,  it  is  also  customary  to  make  a  further 
charge,  which  is  in  the  nature  of  a  penalty,  according  to  the 
maximum  number  of  kilowatts  or  kilovolt-amperes  by  which 
the  rated  capacity  has  at  any  time  been  exceeded  during 
a  week,  month,  or  other  period  of  time ;  this  excess  de- 
mand is  based  upon  the  maximum  average  K.W.  or  K.V.A. 
measured  over  consecutive  quarter  or  half-hour  intervals.  The 
consumer  in  this  case  is  penalised  for  a  demand  exceeding  the 
stipulated  amount,  and  does  not  receive  any  rebate  should  the 
maximum  demand  similarly  measured  be  less  than  the  fixed 
amount  chargeable  on  the  rated  plant  capacity  ;  in  this  latter 
respect  the  payment  differs  from  that  made  according  to  a  true 
maximum  demand  rate. 

Maximum  Demand  Rate. — This  rate  is  very  similar  to  the 
flat  rate,  and  is  charged  to  the  consumer  in  order  to  cover  the 
fixed  charges  on  the  central  station  plant  kept  available  for 
supplying  the  consumer  with  power. 

The  flat  rate  charge  is  based  on  the  estimated  demand  in 
K.W.  or  K.V.A.,  which  obviously  cannot  be  favourable  to  the 
consumer,  who,  in  some  cases,  may  be  further  penalised  for  any 
demand  exceeding  the  agreed  figure.  The  maximum  demand 
rate  is  based  upon  actually  measured  maximum  demands  in 
K.W.  or  K.V.A.,  so  that  the  charge  payable  by  the  consumer  is 
more  strictly  proportional  to  the  maximum  amount  of  power 
demanded  from  and  supplied  by  the  Power  Co.  during  a  given 
period  of  time.  The  maximum  demand  is  usually  recorded  by 
a  special  watt-hour  meter,  first  introduced  by  Merz.  The  instru- 


POWER   CONSUMPTION   COST  AND   CONTRIBUTORY  FACTORS      105 

inent  records  the  maximum  of  the  average  number  of  kilowatts 
demanded  during  any  one  of  consecutive  quarter  or  half-hour 
periods,  which  are  determined  by  a  clock  actuating  the  record- 
ing mechanism  at  regular  intervals.  The  fixed  charge  is  then 
made  upon  this  maximum  demand  at  an  agreed  rate,  and  is 
usually  chargeable  on  each  monthly  or  three-monthly  demand, 
according  to  the  terms  of  the  contract. 

The  maximum  demand  rate  of  payment  is  seldom  used  to- 
gether with  a  meter  rate,  as  it  is  generally  alone  intended  to 
cover  the  actual  generating  cost  of  power.  This  combined 
meter  and  maximum  demand  rate  of  charge  (sometimes  known 
as  the  Hopkinson  two-charge  rate)  is  now  almost  universally 
adopted  both  in  England  and  in  America  for  electric  furnace  in- 
stallations, and  provides  a  most  equitable  basis  for  power  con- 
tracts. When  the  supply  of  power  is  contracted  for  by  Power 
Companies  with  small  generating  stations,  there  is  usually  a 
supplementary  clause  by  which  the  consumer  agrees  to  pay  a 
fixed  annual  payment,  and  to  indemnify  the  Power  Co.  in  the 
event  of  the  premature  termination  of  the  agreement.  Such 
fixed  annual  payments  are  usually  based  on  a  demand  figure 
rather  less  than  the  rated  capacity  of  the  installations,  so  that, 
even  in  this  case,  the  consumer  will  benefit  by  paying  the 
kilowatt  or  kilovolt-ampere  charge  on  the  maximum  demand 
rather  than  on  the  true  flat  rate  basis. 

Load  Factor. — Whenever  the  flat  rate  or  maximum  demand 
rate  systems  enter  into  a  power  contract,  it  is  evident  that  the 
consumer  shoulders  the  burden  of  stand-by  losses  occasioned  by 
an  intermittent  demand  on  the  generating  plant.  Kegarding 
such  plant  as  capable  of  producing  a  profit  to  the  consumer 
while  in  operation,  it  is  obviously  to  his  advantage  to  make  full 
use  of  this  profitable  asset,  and  so  reduce  the  proportion  of 
fixed  charges  on  the  plant  relative  to  the  running  costs. 

A  power  bill  may  be  considered  as  consisting  of  two  items, 
namely,  a  fixed  charge  and  a  running  charge,  which  are  met  by 
the  maximum  demand  or  flat  rate  payment  and  the  meter  rate 
payment  respectively.  The  actual  cost  to  the  consumer  per 
B.O.T.  unit  will  vary  according  to  the  total  units  consumed 
during  a  certain  period,  or  rather  to  the  load  factor,  as  indicated 
by  the  curve  shown  in  Fig.  77.  This  curve  is  plotted  from 


106 


THE   ELECTEO-METALLUEGY   OF   STEEL 


the  calculated  cost  per  unit  taken  over  one  month  at  different 
load  factors.  In  this  case  it  has  been  supposed  that  the 
maximum  demand  is  constant  at  600  kw.  every  month,  and 
charged  at  the  rate  of  7s.  per  month  for  every  kilowatt  demanded, 
while  a  meter  rate  of  *3d.  per  B.O.T.  unit  is  charged  on  the  total 
units  consumed  during  the  year. 

The  load  factor  of   very  many  electric  furnaces  is  below 

MOMTHL.V     DEMAND  -  6OO  K.  Ws  (p   7J-  K  W 


ME.TE.R    RATE    CHARGE 


.  -3V» 


3"   PER.  UNIT 


1O  20          30         40          50          6O          7O          8O          9O         IOO 

PERCENTAGE      LOAD    FACTOR   (MOMTMUY} 

FIG.  77. 

40  per  cent.,  taken  over  a  whole  year,  and  it  will  be  seen  that 
it  is  below  this  point  that  the  curve  begins  to  rise  rapidly. 
This  clearly  shows  the  great  saving  of  power  cost  to  be 
gained  by  even  a  small  improvement  in  load  factor.  The 
curve  shows  a  reduction  in  the  cost  per  unit  of  15 '5  per 
cent.,  resulting  from  an  increase  of  the  load  factor  from  30  per 
cent,  to  40  per  cent. 


POWEK   CONSUMPTION   COST  AHD   CONTEIBUTOEY  FACTOKS      107 

The  actual  load  factor  of  a  highly  efficient  steel  furnace 
installation,  working  day  and  night  and  the  usual  five  and  a  half 
day  week,  was  found  to  be  46  per  cent.  This  figure  covered  a 
period  of  nine  consecutive  months'  operation,  which  included 
the  total  time  covered  by  all  repairs  and  usual  holidays,  so  that 
under  such  conditions  40  per  cent,  may  be  taken  as  a  fail- 
average  figure  for  the  purpose  of  estimating  the  actual  over-all 
cost  of  power  per  unit  according  to  different  contracts. 

So  far  the  effect  of  load  factor  has  only  been  considered 
in  its  relation  to  the  actual  cost  of  power,  whereas  its  influence 
upon  output  and  reduction  of  manufacturing  charges  is  of  still 
greater  importance. 

The  total  manufacturing  cost  per  ton  of  steel  may  be  broadly 
divided  between  a  fixed  charge  on  plant  and  administration 
and  the  actual  cost  of  manufacture,  which  includes  all  materials, 
labour,  power,  and  sundry  charges.  Of  these  two  distinct 
charges  the  fixed  charge  is  distributable  over  the  actual  output 
in  a  given  time,  and  obviously  becomes  less  per  ton  as  the  out- 
put increases.  The  running  cost  is  likewise  reduced  owing  to 
reduced  power  cost,  and  lower  labour  and  repair  charges.  With 
regard  to  the  two  latter  charges  it  does  not  follow  that  an  in- 
creased output  entails  a  shorter  life  of  furnace  lining,  in  fact, 
experience  often  proves  the  opposite ;  also,  an  increased  out- 
put can  usually  be  obtained  without  increasing  the  number  of 
furnace  hands.  The  nett  result,  therefore,  of  improving  the 
load  factor  is  to  produce  an  increased  output  at  lower  cost,  and 
so  doubly  increase  the  net  profit  per  ton  of  steel  made. 

Maximum  Demand. — As  has  been  previously  stated,  the 
maximum  demand  is  the  figure  upon  which  the  fixed  rate  power 
charge  is  made  in  order  to  cover  all  standing  charges  on  the 
power  plant.  The  maximum  demand  figure  should  be  fixed 
and  regarded  by  the  consumer  as  a  power  demand  which  it  is 
not  intended  to  exceed,  and  may  be  taken  to  represent  the 
maximum  permissible  power  capacity  of  the  installation.  This 
being  the  case  it  may  be  considered  almost  as  a  flat  rate  charge 
based  upon  a  definite  estimated  demand. 

Since  load  factor,  averaged  over  a  period  of  a  year  or  even 
over  the  period  of  a  single  heat,  is  such  an  all-important  element 
in  the  economy  of  manufacture,  it  is  of  great  importance  to  take 


108  THE   ELECTKO-METALLTJKGY   OF   STEEL 

full  advantage  of  the  permissible  power  demand  at  all  times, 
consistent  only  with  the  varying  metallurgical  and  electrical 
conditions  required  during  the  furnace  operation.  The  effect 
of  load  factor  during  a  single  heat  is  also  very  considerably 
greater  than  is  at  first  apparent,  and  is  explained  at  greater 
length  in  connection  with  furnace  radiation  loss  and  power 
input.  The  maximum  demand,  although  actually  recorded  as 
K.W.  by  the  integrating  watt-hour  meter  attachment  previously 
referred  to,  is  usually  converted  to  K.V.A..  for  purposes  of  a 
maximum  demand  rating  based  on  K.V.A.  instead  of  K.W.  For 
this  conversion  it  is  necessary  to  know  the  average  power  factor 
over  the  entire  period  for  which  the  maximum  demand  is  re- 
corded ;  this  figure  is  ascertained  from  the  combined  readings 
of  a  kilowatt-hour  meter  and  of  an  integrating  meter  which 
measures  the  wattless  component  of  the  load.  The  maximum 
demand  recorded  in  K.W.  can  then  be  converted  into  K.V.A.  by 
using  the  average  power  factor  so  determined  as  a  multiplier. 

It  should  also  be  recognised  that,  if  the  maximum  demand 
should  for  any  reason  exceed  the  normal  figure  during  one- 
quarter  or  one-half  hour  interval,  the  cost  of  power  per  unit 
during  the  whole  of  the  period  charged  for  will  be  increased, 
which  merely  corresponds  to  the  effect  of  a  reduced  load  factor. 

Furnace  Radiation  Loss  and  Power  Input. — The  power 
absorbed  in  an  electric  furnace  is  not  all  usefully  used  in  sup- 
plying heat  energy  to  a  furnace  charge,  a  considerable  portion 
being  constantly  dissipated  as  radiated  heat  according  to  the 
difference  in  temperature  between  the  inner  and  outer  surfaces 
of  the  furnace  lining,  its  thermal  conductivity,  and  other  factors. 
The  consequent  effect  of  this  ratio  of  useful  to  useless  energy 
was  not  properly  recognised  in  the  earliest  electric  steel  furnace 
installations  in  which  the  proportion  was  about  equal,  and  for 
this  reason  the  power  consumption  was  inordinately  high,  and 
the  lining  repairs  correspondingly  heavy. 

The  radiation  loss  may  be  conveniently  recorded  as  the 
power  in  K.W.  that  is  in  practice  found  to  be  required  to  maintain 
a  constant  temperature  of  a  bath  of  steel  over  a  sustained  period. 
This  figure  can  be  easily  determined  for  every  furnace,  and  will, 
of  course,  vary  with  the  condition  of  the  lining  and  other 
obvious  factors. 


POWER   CONSUMPTION   COST  AND   CONTRIBUTORY  FACTORS      109 

In  most  furnaces  of  3  tons  capacity  the  radiation  loss,  as 
expressed  in  the  above  terras,  will  be  approximately  equivalent 
to  190  to  220  kw.  ;  this  being  so,  there  is  not  such  a  very  con- 
siderable margin  of  power  available  for  melting  purposes,  even 
where  a  normal  full  load  of  600  kw.  is  provided.  During  the 
earlier  melting  down  period  the  radiation  loss  will  not,  of  course, 
be  so  great,  owing  to  the  lower  temperature  of  the  furnace 

MAXIMUM         INPUT  6OO  K.Yfs 

RADIATION      Loss I6O 


80 


60 


20    30     4O    SO    60    7O     8O    0O    lOO 

Load    factor. 


FIG.  78. 


interior,  but  may  be  estimated  at  160  kw.  for  the  purpose  of 
examining  the  effect  of  load  factor  from  this  particular  stand- 
point during  the  melting-down  period  of  a  heat.  The  curve 
shown  in  Fig.  78  illustrates  how  the  percentage  of  useful  energy 
in  a  given  power  input  varies  for  different  values  of  load  factor, 
assuming  an  average  radiation  loss  of  160  kw.  and  a  maximum 
possible  load  input  of  600  kw.  The  excessive  waste  of  energy 


110 


THE    ELECTRO-METALLURGY   OF    STEEL 


is  in  this  way  clearly  indicated,  and  emphasises  the  necessity  of 
operating  a  furnace  at  the  highest  average  load  permissible. 

Another  way  of  illustrating  the  general  influence  of  load 
factor  is  by  plotting  a  curve  that  shows  the  variation  in  power 
consumption  per  ton  of  steel  at  different  load  factors.  To  draw 
such  a  curve  an  actual  case  must  be  taken  from  which  the 
theoretical  units  for  melting  and  refining  a  ton  of  steel  may  ap- 
proximately be  deduced.  The  curve  A  shown  in  Fig.  79  has 
been  plotted  from  figures  calculated  from  a  specific  case,  in 
which  3'2  tons  of  melted  and  refined  basic  steel  were  produced 
in  six  hours  using  2453  units.  Allowing  fifteen  minutes  for 


TE  

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r  Melhoi 

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e>  Hour* 
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L- 

600 

PERCENTAGE    LOAD  FACTOR.(OERAT.NcO 
FIG.  79. 

sundry  delays,  the  average  power  input  was  426  kw.  The  radia- 
tion loss  was,  on  the  other  hand,  continuous  and  constantly  in- 
creasing during  the  full  period  of  six  hours,  and  has  been  estimated 
for  the  purpose  of  this  example  as  averaging  170  kw.  throughout. 
On  this  assumption  the  units  used  usefully  for  melting  and 
refining  purposes  amount  to  450  per  ton  of  steel,  which  agrees 
fairly  well  with  the  generally  accepted  figure.  The  maximum 
permissible  load  was  580  kw.,  which  gives  in  this  case  an 

,  426  x  100 
average  load  factor  of      —  ^r  --  =  73'5  per  cent,    during  the 


five  and  three-quarter  hours  of  actual  operation. 


POWER   CONSUMPTION    COST   AND   CONTRIBUTORY   FACTORS      111 

It  must  be  borne  in  mind  that  as  the  load  factor  decreases 
and  so  lengthens  the  period  of  each  heat  the  average  radiation 
loss  will  be  somewhat  greater  than  170  kw.,  and  so  give  power  con- 
sumption figures  slightly  higher  than  those  indicated  by  the 
curve.  Besides  this,  there  are  other  minor  factors  which  should 
really  be  considered,  but  their  introduction  would  only  slightly 
modify  and  tend  to  obscure  the  result  by  the  many  complica- 
tions involved.  The  curve  B  shows  still  more  directly  the  in- 
fluence of  the  operating  load  factor  on  output,  allowing  thirty-six 
minutes  for  charging  and  fifteen  minutes  delay  during  each  heat, 
which  is,  of  course,  good  practice  but  perfectly  possible  for  this 
size  of  furnace. 

Such  curves  can  be  plotted  for  any  furnace,  provided  the 
average  radiation  loss  is  based  upon  an  observed  figure  at  a 
medium  bath  temperature,  and  the  maximum  permissible  load 
fixed  at  a  given  figure  for  the  purpose  of  determining  the  load 
factors  corresponding  to  different  average  loads. 

Load  Fluctuation. — The  effect  of  load  fluctuation  on  power 
costs  resolves  itself  purely  into  a  question  of  load  factor  and 
output,  as  apart  from  the  objectionable  results  of  a  badly 
fluctuating  load  upon  the  electrical  equipment,  there  is  always 
the  difficulty  of  maintaining  a  desired  average  load.  Load 
adjustment  is  almost  universally  effected  in  accordance  with  the 
indications  of  ammeters,  which,  owing  to  the  rapid  oscillations 
of  their  pointers,  fail  to  indicate  the  average  load  on  the  furnace  ; 
this  inevitably  leads  the  furnace  operators  to  underload  the 
furnace,  so  as  to  avoid  the  heavy  apparent  overloads  inaccurately 
indicated  by  undamped  instruments. 

Everything,  therefore,  should  be  done  in  the  way  of  furnace 
manipulation  to  secure  a  steady  load,  which  can  then  be  more 
easily  maintained  at  the  highest  desirable  figure.  The  character 
of  cold  scrap  used  may  have  a  considerable  influence  on  the 
steadiness  of  the  load  and  operating  load  factor,  so  that  those 
physical  qualities  conducive  to  the  maintenance  of  good  loads 
during  the  melting  period  should  on  no  account  be  sacrificed  for 
the  sake  of  the  apparent  economy  to  be  gained  by  the  purchase 
of  cheap  unsuitable  scrap.  Equal  attention  should  be  paid  to 
the  manner  of  charging  in  order  to  secure  the  best  possible 
electrical  conductivity  within  the  charge ;  observations  on  this 
point  have  been  previously  made. 


112  THE    ELECTRO-METALLURGY   OF    STEEL 

The  introduction  of  considerable  reactance  into  the  load 
circuit  is  undoubtedly  the  most  effective  way  of  damping  out 
fluctuations,  and  so  improving  the  load  factor.  Unfortunately^ 
this  can  only  be  done  at  the  expense  of  power  factor,  which,  if 
any  really  marked  benefit  is  to  be  obtained  from  the  use  of 
reactance  coils,  falls  below  the  usually  guaranteed  average  figure 
of  '8  to  '85  at  normal  full  load. 

Power  Consumption. — It  will  be  apparent,  after  considera- 
tion of  the  above-mentioned  factors,  that  the  units  consumed  in 
melting  and  refining  cold  charges  are  subject  to  considerable 
variation,  which  in  a  large  measure  accounts  for  the  very 
diverse  figures  obtained  with  electric  furnaces  of  either  similar  or 
different  types.  It  has  been  found  by  careful  calorimetric 
determinations  that  the  heat  contained  in  molten  steel  of  average 
casting  temperature  is  equivalent  to  about  370  kw.-hours  per 
ton.  After  making  due  allowance  for  the  normal  extent  of 
chemical  refining  on  a  basic  hearth,  fusion  of  fluxes,  and  other 
minor  power-absorbing  functions,  it  is  not  unlikely  that  the 
above  figure  rises  to  400  or  450  kw.-hours  per  ton  of  finished 
steel  melted  and  refined.  This  figure,  then,  represents  the 
useful  energy  input  required  per  ton  of  steel  produced,  irrespective 
of  the  type  of  furnace,  operating  load  factor,  and  other  conditions. 
The  curve  A  in  Fig.  79  illustrates  how  this  theoretical  figure 
can  easily  rise  to  1100  units  under  adverse  conditions  of  operation 
which  lead  to  a  poor  load  factor,  even  though  the  radiation  loss 
is  not  beyond  a  reasonable  figure.  For  these  reasons,  little 
value  can  be  attached  to  any  power  consumption  figures  other 
than  those  that  indicate  the  best  performance  possible  under 
conditions  that  should  be  clearly  stated. 

A  good  average  power  consumption  resulting  from  con- 
tinuous operation  may  be  taken  as  750  kw.-hours  per  ton  of 
steel  produced,  but  it  must  be  understood  that  this  figure  is  subject 
to  considerable  reduction  or  increase  according  to  the  degree  of 
manipulative  skill,  the  process  employed,  and  the  ratio  of  useful 
energy  input  to  radiation  loss. 

When  using  the  acid  process  the  power  consumption  will 
usually  be  somewhat  lower  than  for  the  basic,  other  conditions 
being  similar. 


CHAPTER  VII. 

ELECTRO-METALLURGICAL  METHODS  OF  MELTING  AND 
REFINING  COLD  CHARGES. 

Introduction. — The  electric  furnace  is  used  in  steel  works  to 
perform  a  variety  of  functions.  Generally  speaking,  the  smaller 
units  serve  to  perform  all  the  metallurgical  operations  for  the 
production  of  finished  steel  from  a  crude  charge  of  miscellaneous 
steel  or  iron  scrap.  Furnaces  of  the  largest  capacity  are  usually 
employed  in  conjunction  with  other  steel-making  plant,  and  are 
then  used  to  perform  one  or  sometimes  two  distinct  operations 
of  a  process  conducted  in  several  stages.  In  either  case  the 
furnaces,  which  may  be  basic  or  acid  lined,  are  suitable  for 
carrying  out  operations  which  are  common  to  the  open  hearth 
furnace,  but  in  a  modified  manner.  Owing  to  the  nature  of 
electric  heating,  which  enables  chemical  reactions  to  proceed 
in  an  atmosphere  uncontaminated  with  oxidising  gases  and 
under  slags  of  special  character,  operations  may  also  be  per- 
formed which  have  no  counterpart  in  a  gas  or  any  other  type 
of  furnace. 

The  several  processes  used  in  the  manufacture  of  steel,  made 
either  wholly  or  in  part  in  electric  furnaces,  must  be  broadly 
classified  according  to  the  acid  or  basic  character  of  the  slag 
employed.  Each  process  must  also  be  studied  separately  in  its 
application  to  the  treatment  of  cold  scrap  and  liquid  steel. 

The  basic  process  has  been  most  generally  used  for  the  pro- 
duction of  finished  steel  from  cold  charges,  while  the  acid 
process  affords  special  advantages  for  foundry  practice  and  for 
finishing  semi-refined  liquid  steel.  The  basic  process,  as  applied 
to  the  open  hearth  or  Bessemer  converter,  is  far  more  limited 
in  its  application  than  when  practised  in  the  electric  furnace. 
Since  electric  heating  can  be  applied  to  a  furnace  charge  in  such 
a  manner  as  to  exclude  all  oxidising  gases,  it  is  not  only  possible 

(113)  8 


114  THE   ELECTRO-METALLURGY  OF   STEEL 

to  maintain  a  reducing  atmosphere,  but  also  a  powerfully  re- 
ducing slag,  exceedingly  basic  or  limey  in  character,  which  has 
power  both  to  deoxidise  and  desulphurise  a  bath  of  semi-finished 
steel.  This  property  of  deoxidation  is  entirely  absent  in  open 
hearth  and  converter  slags,  which  contain  at  least  16  per  cent, 
of  FeO,  and  whose  range  of  chemical  action  is  limited  to  one  of 
oxidation  alone.  It  is  true  that  a  basic  open  hearth  slag,  to 
which  has  been  added  a  quantity  of  calcium  chloride  according 
to  the  Saniter  process,  has  a  certain  power  of  desulphurising,  but 
the  degree  of  sulphur  removal  does  not  equal  the  extraordinarily 
low  figures  obtainable  with  electric  furnace  basic  slags. 

With  regard  to  the  acid  process,  there  is  a  far  closer  identity 
of  behaviour  of  the  acid  slags  used  in  converters  and  in  electric 
and  open  hearth  furnaces,  as  in  each  case  the  slag  will  only 
remove  carbon,  silicon,  and  manganese,  without  any  reduction 
of  sulphur  and  phosphorus.  In  certain  instances,  the  acid  slag 
in  an  electric  furnace  may  be  partially  reduced  by  carbon,  when 
it  then  helps  to  accelerate  the  deoxidation  of  the  bath ;  this 
applies  more  especially  to  liquid  refining.  In  this  latter  respect 
it  differs  from  open  hearth  and  converter  slags,  which  are  not 
subjected  to  the  same  intense  local  heat. 

The  general  principles  governing  the  selection  of  one  or 
other  process  for  use  in  the  electric  furnace  are  different  from 
those  which  might  be  applied  in  the  case  of  the  open  hearth 
or  converter.  In  the  latter  cases,  apart  from  the  consideration 
of  available  pig-iron,  the  quality  of  steel  required  usually  defines 
the  choice  of  process.  Acid  steel  made  from  high  class  raw 
materials  is  admittedly  superior  in  quality  to  basic  steel  of 
similar  composition,  but  this  does  not  apply  to  electric  furnace 
steel  and,  if  anything,  is  the  reverse.  It  is  now  generally 
accepted  that  basic  electric  steel,  if  properly  made,  meets  all 
the  requirements  of  crucible  steel,  whether  it  be  plain  carbon, 
simple  alloy  or  high  speed,  but  this  is  only  possible  if  the  phos- 
phorus and  sulphur  are  exceedingly  low  and  at  a  figure  which 
acid  electric  steel,  made  even  from  a  good  class  of  steel  scrap, 
could  not  approach. 

The  acid  electric  process,  however,  has  been  used  largely  for 
finishing  semi-refined  steel.  In  this  respect,  the  furnace  merely 
serves  as  a  convenient  internally  heated  receptacle,  in  which 


METHODS   OF   MELTING   AND   REFINING   COLD   CHARGES      115 

oxidised  steel  may  be  brought  up  to  a  casting  temperature  and 
deoxidised  under  slightly  reducing  or  neutral  conditions.  Neces- 
sary additions  of  alloys  may  be  also  conveniently  made  prior 
to  casting.  Of  recent  years  considerable  attention  has  been 
directed  to  the  beneficial  effects  derived  by  holding  steel  in  a 
perfectly  tranquil  state  for  a  short  period  before  teeming.  This 
procedure  enables  minute  slag  particles  in  suspension,  and 
possibly  gases,  to  rise  through  the  steel  and  escape.  The  con- 
ditions required  to  promote  this  simple  physical  action  are 
perfectly  fulfilled  by  the  electric  furnace,  and  it  is  probably 
some  such  secondary  effect,  proceeding  simultaneously  with  the 
process  of  deoxidation,  that  helps  to  impart  the  high  qualities 
to  carefully  refined  acid  steel.  This,  of  course,  equally  applies 
to  basic  electric  steel. 

Owing  to  the  extravagant  cost  of  electric  energy  as  a 
source  of  heat,  metallurgical  operations,  which  may  be  satis- 
factorily and  economically  performed  in  either  the  open  hearth 
furnace  or  converter,  are  at  once  precluded  from  electric  furnaces. 
Hence  the  electric  furnace  is  used  only  where  the  others  fail, 
and  is  therefore  usually  limited  to  the  further  refining  and 
finishing  of  comparatively  pure  raw  materials.  The  charge 
may  equally  well  be  cold  or  liquid,  but  in  either  case  will  not 
require  more  than  a  small  degree  of  purification.  Although  the 
actual  reduction  of  impurities  may  be  very  slight,  it  is  just  that 
final  degree  of  refining  which  justifies  the  use  of  the  electric 
process.  As  a  general  rule,  therefore,  it  may  be  assumed  that 
the  charge  will  consist  of  liquid  steel  or  steel  scrap,  and  average 
only  a  small  percentage  of  carbon,  phosphorus  and  sulphur. 
Excess  of  the  two  former  elements  may  prolong  the  period  of 
chemical  action  considerably  beyond  that  of  melting,  in  which 
case  the  economic  advantage  of  the  electric  process  will  be 
seriously  impaired. 

BASIC  PROCESS. 

This  process,  as  usually  conducted,  may  be  briefly  divided 
into  three  distinct  stages  : — 

(i)  Melting  down  under  oxidising  conditions. 

(ii)  Skimming  and  carburising. 

(iii)  Kefining  under  powerfully  reducing  conditions  and 
finishing  with  alloy  additions,  etc. 


116  THE   ELECTRO-METALLURGY   OF    STEEL 

General  Outline. — The  process,  as  conducted  in  the  various 
types  of  basic  lined  furnaces,  either  with  or  without  conductive 
hearths  or  bottom  electrodes,  is  the  same  in  general  principle. 
The  details  of  operation,  more  especially  charging,  may  be 
slightly  different,  but  the  method  of  working  will  not  deviate 
far  from  the  following  description,  which  applies  more  strictly 
to  direct  arc  furnaces  without  conductive  hearths.  For  the 
operation  of  indirect  arc  furnaces,  slight  modifications  in 
methods  of  charging  will  suggest  themselves,  and  are  here 
purposely  omitted  so  as  not  to  confuse  or  destroy  the  continuity 
of  the  following  description  of  the  process.  The  various  steps 
in  the  mechanical  operation,  together  with  the  chemical  changes 
that  occur,  may  be  briefly  summarised  in  consecutive  order  : — 

1.  Hand,  or  mechanical  charging  of  fluxes  and  scrap  into 
the  furnace,  which  has  been  previously  heated. 

2.  Load  put  on  to  the  furnace  and  melting  begins. 

3.  Scrap  melts  under  the  electrodes,  which  bore  downwards 
until  pools  of  metal  are  formed  on  the  bottom  with  a  slag 
covering. 

4.  Melting    and    further  slag  formation  proceeds  until  all 
fluxing  materials  in  the  original  charge  are  fused. 

5.  A  considerable  portion   of  the  scrap  melted,  forming  a 
bath  ;  appearance  of  slag  observed,  and  additions  of  ore  or  lime 
made  if  necessary.      Feed  of  scrap   given,    filling   furnace   as 
far  as  possible.     Eemoval  of   carbon,  manganese,  silicon,  and 
phosphorus  proceeds,  provided  the  slag  is  sufficiently  oxidising 
and  basic. 

6.  Second  feed  given.     Chemical  reactions  still  proceed. 

7.  All  the  charge  melted.     Bath  hot  and  showing  very  slight 
boil.     Slag  appearance  correct. 

8.  Load  off,  skimming  started. 

9.  Bath  skimmed.      Carbon  additions  made  to  naked  bath 
if  necessary,  followed  sometimes  by  a  small  addition  of  ferro- 
silicon,  and  then  a  mixture  of  fluor  spar  and  lime.     Bath  be- 
comes partly  deoxidised  by  the  ferro- silicon  addition. 

10.  Load  on.     Fluxes  begin  to  fuse  and  correct  slag  forma- 
tion is  promoted  by  an  addition  of  carbon  dust.     Bath  fairly  hot. 

11.  Slag   fused  ;    bath  well  rabbled.     Slag   pale   in    colour. 
Bath  sample  taken  for  analysis,  if  required.     Slag  thickened  by 


METHODS   OF   MELTING  AND  EEFINING  COLD   CHAEGES      117 

further  lime  additions.  Carbon  dust  added  in  small  quantities, 
as  required,  to  complete  the  removal  of  metallic  oxides  from  the 
slag,  and  to  promote  the  formation  of  calcium  carbide. 

12.  Sample  taken  and  a  small  addition  of  ferro-silicon  made, 
if  necessary.     Further  deoxidation  of  the  bath  proceeds,  and 
if  the  slag  is  nearly  or  quite  white,  desulphurisation  begins. 

13.  Heat  tests  and  bath  samples  taken  from  time  to  time 
until  the  steel  lies  quiet  in  a  sample  mould.     Deoxidation  and 
desulphurising  action  finished. 

14.  Bath  at   casting   temperature ;  alloy  additions  made  if 
required.     Bath   rabbled  after   five  minutes,  and  final  sample 
taken. 

15.  Load  off.     Steel  poured. 

The  above  summary  must  not  be  regarded  as  a  strictly  ac- 
curate history  of  a  basic  heat,  it  being  obvious  that  the  chemical 
changes  are  progressive  and  gradual,  and  overlap  the  manipu- 
lative operations.  It  is  only  intended  to  assist  in  piecing  to- 
gether the  detailed  description  of  the  different  phases  of  the 
process,  and  to  serve  as  a  guide  for  examining  the  relationship 
between  the  various  operations  and  the  chemical  reactions  which 
they  produce. 

Choice  of  Scrap. — The  selection  of  scrap  has  an  important 
bearing  on  the  economic  operation  of  any  electric  steel  furnace. 
In  many  cases  it  may  be  impossible  to  use  a  type  of  scrap  that 
is  suitable  in  every  way,  but  careful  judgment  in  the  selection 
and  mixing  of  inferior  grades  may  lead  to  equally  satisfactory 
results,  not  only  as  regards  the  quality,  but  also  the  quantity  of 
steel  made.  It  should  be  remembered  that  chemical  analysis  is 
not  the  only  standpoint  from  which  the  value  of  scrap  can  be 
judged,  since  the  shape  and  size  have  considerable  influence  on 
the  electrical  conditions  during  melting.  A  suitable  class  of 
scrap  should  be  such  as  will  give  no  trouble  with  the  metallurgi- 
cal operations,  or  in  the  maintenance  of  a  steady  electrical  load. 
The  employment  of  light  scrap  of  irregular  shape  or  very  light 
turnings  is  sure  to  cause  considerable  difficulties  in  maintaining 
a  full  and  steady  load,  unless  judiciously  mixed  with  some  class 
of  heavy  scrap,  which  may  help  to  eliminate  the  disadvantages 
otherwise  experienced.  An  unsteady  and  low  load,  due  to  the 
irregular  resistance  of  such  scrap,  results  in  a  poor  load  factor, 


118  THE    ELECTKO-METALLUKGY   OF    STEEL 

which  in  itself  is  a  fundamental  cause  of  poor  output,  thermal 
inefficiency,  and  increased  cost  of  production.  It  must  be  re- 
membered that  electrical  energy  equivalent  to  the  radiation  loss 
of  the  furnace  constitutes  a  large  percentage  of  the  maximum 
power  input  of  the  furnace  (more  especially  in  the  smaller  sizes), 
and,  therefore,  inability  to  maintain  the  power  available  for  useful 
heating  at  its  maximum  results  in  considerable  loss  of  time  and 
failure  to  operate  the  plant  at  its  full  capacity. 

The  nature  of  the  scrap  from  a  chemical  standpoint  is  also 
important.  The  phosphorus  and  sulphur  contents  should  be 
reasonably  low,  so  as  not  to  unduly  prolong  the  period  of  the 
refining  action  of  the  basic  oxidising  and  reducing  slags.  Gener- 
ally speaking,  almost  any  class  of  steel  scrap  is  sufficiently  low 
in  these  elements  and  will  be  satisfactory,  provided  the  percent- 
age of  carbon  is  also  low.  Carbon,  either  chemically  combined 
or  mechanically  mixed  in  the  form  of  coke,  cinders,  or  oil,  affects 
the  suitability  of  ordinary  steel  scrap  more  than  any  of  its  other 
chemical  constituents,  Coke  and  cinders  are  very  commonly 
found  in  turnings  which  have  been  dumped  on  to  freshly  made 
ground,  but  of  all  forms  of  carbonaceous  foreign  matter,  the 
worst  is  oil.  It  is  generally  inadvisable  to  melt  oily  turnings 
as  the  sole  constituent  of  a  charge ;  the  action  of  heat  decom- 
poses the  oil,  which  leaves  a  fine  deposit  of  carbon  on  the  turn- 
ings, the  greater  part  of  which  is  absorbed  and  passes  into  the 
bath  of  steel.  For  this  reason,  the  bath,  when  otherwise  ready 
to  skim,  will  contain  too  much  carbon,  which  must  then  be 
boiled  out  to  ensure  complete  removal  of  phosphorus.  When  a 
charge  consists  of  low  carbon  scrap,  the  process  of  decarburising 
and  dephosphorising  should  proceed  as  fast  as  the  melting 
operation,  so  that  when  all  the  charge  is  melted  and  hot  enough 
for  skimming,  the  carbon,  phosphorus,  manganese,  and  silicon 
contents  will  be  as  ]ow  as  required.  This  is  not  always  possible 
when  the  scrap  is  over-charged  with  carbon,  owing  to  the  diffi- 
culty of  maintaining  a  strongly  oxidising  basic  slag  during 
melting.  The  metallic  oxides  are  rapidly  reduced  and  the  con- 
ductivity of  the  slag  is  thereby  lowered  ;  in  order,  then,  to 
maintain  a  full  load  current  the  electrodes  may  be  forced  to  dip 
into  the  slag,  and  this  under  certain  conditions  gives  rise  to 
heavy  current  fluctuations.  The  electrodes,  again,  should  never 


METHODS   OF   MELTING   AND   REFINING   COLD   CHARGES      119 

under  any  circumstance  dip  into  an  oxidising  slag,  as  any  addi- 
tion of  ore  is  rapidly  reduced  by  the  action  of  the  carbon  elec- 
trodes rather  than  by  the  action  of  the  carbon  in  the  bath. 
These  facts  demonstrate  the  excessive  waste  of  time  involved 
in  boiling  down  carbon  and  ensuring  the  removal  of  phosphorus, 
which  can  only  be  accomplished  in  extreme  cases  by  frequent 
skimmings  and  constant  additions  of  ore.  The  use  of  light, 
stringy  turnings  should  also  be  avoided,  as  the  loss  of  time  and 
heat  occasioned  by  the  frequent  charging  of  such  light,  bulky 
scrap  usually  represents  considerably  more  money  than  the  extra 
cost  of  a  more  suitable  scrap.  It  may  also  be  pointed  out  that, 
when  charging  stringy  turnings  into  a  small  furnace  using 
graphite  electrodes,  there  is  considerable  risk  of  breaking  the 
latter ;  also,  the  melting  loss  by  oxidation  is  excessive. 

The  following  observations  will  serve  as  a  guide  in  the  selec- 
tion of  suitable  scrap  or  mixtures  of  scrap  for  melting  : — 

I.  Scrap  should  be  of  such  size  and  form  as  to  pack  closely 
in  the  furnace  and  render  charging  operations  capable  of  being 
conducted  with  minimum  expenditure  of  time  and  labour,  and 
without  risk  of  damaging  the  door  jambs  or  electrodes. 

II.  Scrap  should  be  sufficiently  heavy  (a)  to  enable  the  en- 
tire charge  to  be  introduced  into  the  furnace  without  more  than 
two  feeds  after  the  initial  charging  ;  (b)  to  form  pools  of  metal 
under  each  electrode  of  sufficient  size  (i)  to  prevent  entire  rup- 
ture of  the  electric  circuit,  which  might  otherwise  occur — more 
particularly   with  small   graphite   electrodes — should  the   elec- 
trodes bore  down  to  the  bottom  and  then  lose  contact  with  the 
unmelted  charge,  (ii)  to  prevent  the  electrodes  arcing  on  to  the 
bottom  in  the  case  of  conductive  hearth  furnaces. 

III.  It  should  be  moderately  free  from  rust  to  prevent  over- 
oxidation  of  the  bath  ;  in  reasonable  amounts  rust  is  useful,  as 
it  will  assist  the  removal  of  carbon  and  maintain  a  slag  charged 
with  oxide  during  melting -operations  without  the  addition  of 
iron  ore. 

IV.  The  use  of  high  carbon  scrap  alone  is  generally  avoided, 
as,  when  melted,  the  resulting  bath  will  probably  contain  carbon, 
and  require  further  boiling  down  with  ore  additions  ;  at   the 
same  time  there  is  danger  of  imperfect  removal  of  phosphorus. 
Dead  soft  steel  or  wrought-iron  scrap  is  also  objectionable  if 


120  THE    ELECTEO-METALLUBGY   OF    STEEL 

melted  alone.  The  bath  is  invariably  over-oxidised  and  cold 
melted,  and  the  erosion  of  the  dolomite  banks  and  wear  on  the 
roof  and  walls  are  considerably  increased.  The  best  average 
carbon  content  of  a  charge  of  scrap  is  from  '3  per  cent,  to  '4  per 
cent. 

V.  The  presence  of  carbon  in  scrap  as  foreign  matter,  such 
as  unconsumed  cinder  and  especially  oil,  has  already  been  dealt 
with. 

VI.  Wet  scrap  is  objectionable  as  it  is  liable  to  cause  ex- 
plosions when  fed  into  a  bath  with  a  slag  covering  ;  this,  besides 
being  a  source  of  danger  to  the  furnacemen,  impedes  the  rate  of 
charging,  and  consequently  increases  heat  losses  and  reduces 
output.     Wet  turnings  fresh  from  the  machine  shop  should  be 
allowed  to  drain  for  a  week  or  more  before  being  melted. 

VII.  A  high  manganese  content  is  sometimes  imperfectly 
removed — particularly  if  the  carbon  is  also  high. 

It  must  not  be  supposed  that  scrap,  which  does  not  fulfil  all 
the  conditions  enumerated  above,  is  unsatisfactory  ;  as  previ- 
ously stated,  a  judicious  selection  and  mixing  of  grades,  un- 
suitable by  themselves,  is  quite  capable  of  producing  the  very 
best  results  both  as  regards  quality  and  output. 

Method  of  Charging  Scrap. — Scrap  should  invariably  be 
charged  in  a  manner  that  will  offer  the  least  resistance  to  the 
passage  of  current  through  the  charge.  Kail  and  bar  ends  and 
other  such  shapes  should  be  charged  side  by  side  as  closely  as 
possible,  and  not  in  criss-cross  fashion.  A  little  extra  time  and 
trouble  spent  in  careful  charging  will  often  save  considerable 
loss  of  time  caused  by  a  fluctuating  load.  Heavy  solid  scrap, 
such  as  ingot  heads,  when  mixed  with  very  light  scrap,  should 
be  charged  under  the  electrodes  on  the  bottom  ;  this  will  pre- 
vent any  possibility  of  the  electrodes  boring  down  through 
the  light  scrap  to  the  bottom  and  losing  contact  with  the 
charge. 

The  initial  charge  should  be  of  such  quantity  that  one  or, 
at  the  most,  two  subsequent  feeds  will  suffice  to  complete  the 
charging  operation.  Constant  opening  of  the  charging  doors 
for  this  purpose  is  a  practice  to  be  avoided,  owing  to  the  con- 
siderable loss  of  heat  from  the  interior  of  the  furnace,  more 
especially  when  the  bulk  of  the  charge  is  melted. 


METHODS   OF   MELTING   AND   EEFINING   COLD    CHAKGES      121 

Care  is  always  taken  to  charge  scrap  in  a  manner  that  will 
afford  maximum  protection  to  the  furnace  banks  ;  in  cases  where 
turnings  constitute  a  large  portion  of  the  charge,  it  is  an  easy 
matter  to  effect  this  by  reserving  the  turnings  for  covering  the 
banks  from  the  slag  line  upwards.  In  the  immediate  vicinity 
of  the  arc  the  turnings  will  rapidly  melt,  and  during  subsequent 
feeds  fresh  quantities  should  be  charged,  as  far  as  possible,  be- 
tween the  electrodes  and  the  nearest  portion  of  the  banks.  Un- 
melted  scrap  remaining  on  the  banks  is  left  undisturbed  until 
the  charge  is  almost  ready  for  skimming,  when  it  may  be  gently 
pushed  into  the  bath  with  the  aid  of  a  hooked  bar. 

Irregular  shaped  heavy  scrap,  if  used  in  small  quantities,  is 
preferably  kept  out  of  the  initial  charge  and  added  to  the  bath 
during  subsequent  feeds  to  avoid  disturbance  of  the  furnace 
load. 

Scrap  should  not  be  fed  into  a  furnace  until  the  initial 
charge  has  been  melted  to  form  a  fairly  hot  bath  of  steel  be- 
tween the  electrodes.  The  practice  of  pushing  in  fresh  scrap 
as  soon  as  there  is  any  available  space  leads  to  all  the  troubles 
associated  with  cold  melting.  The  slag,  being  thus  constantly 
chilled,  does  not  promote  the  rapid  elimination  of  carbon  and  the 
metalloids  from  the  cold  bath  of  metal,  as  evidenced  by  a  dead- 
looking  appearance  and  absence  of  boil.  The  bath  of  metal  is 
also  likely  to  chill  on  the  bottom,  and  will  then  require  a  small 
addition  of  pig  for  its  removal,  which  could  otherwise  only  be 
effected  by  prolonged  heating.  There  is  also  the  possibility  of 
the  bath  becoming  unduly  charged  with  dissolved  oxides,  and 
this  adds  to  the  difficulty  of  the  subsequent  carburising  (if  any), 
deoxidising,  and  desulphurising  treatments.  Provided  the  com- 
position of  the  slag  is  correct,  a  slight  boil  will  indicate  that 
the  temperature  of  the  bath  formed  from  the  initial  charge  is 
suitable  for  fresh  additions  of  scrap  to  be  made. 

If  a  furnace  is  properly  charged  and  left  undisturbed  while 
the  load  is  maintained,  the  portion  of  the  charge  lying  between 
the  electrodes  will  frequently,  and  especially  in  the  case  of 
turnings,  frit  or  weld  together  and  ultimately  collapse  into  the 
bath  beneath  it,  causing  a  violent  boil  and  heavy  current  over- 
load. This  usually  occurs  rather  suddenly,  and,  unless  the 
melter  is  on  the  alert,  may  cause  the  automatic  trip  to  operate 


122  THE    ELECTRO-METALLURGY   OF    STEEL 

and  cut  off  the  supply  of  power.  A  skilful  melter  will  usually 
anticipate  such  an  occurrence  by  gently  pushing  that  portion  of 
the  charge  into  the  bath  just  before  it  is  likely  to  collapse. 
This  point  marks  a  favourable  time  for  feeding,  which  may  be 
done  a  few  minutes  after  the  slight  resulting  boil  has  subsided. 
A  furnace  charged  and  fed  in  this  manner  will  not  have  acquired 
a  high  roof  temperature  by  the  time  the  bath  is  ready  for 
skimming.  Some  of  these  remarks  may  not  apply  to,  or  may 
be  modified  to  suit  various  types  of  direct  arc  furnace,  but  the 
general  principles  are  the  same  for  all. 

In  the  case  of  indirect  arc  furnaces,  such  as  the  Stassano, 
care  has  to  be  taken  not  to  break  the  long  high  voltage  arcs 
formed  between  the  electrode  tips  by  throwing  cold  scrap 
directly  across  their  path.  If  only  one  of  the  three  arcs  formed 
in  a  Stassano  furnace  is  broken,  the  other  two  remain  nearly 
unaffected  and  the  load  is  not  entirely  ruptured ;  but  the  re- 
duction of  power  and  the  resultant  out-of-balance  load  should 
be  avoided. 

Basic  Oxidising  Slag. — Function. — In  this  case  the  basic  slag 
will  contain  a  large  percentage  of  iron  oxide  for  the  purpose  of 
eliminating  carbon,  manganese,  silicon,  and  phosphorus  from 
the  bath  of  metal  by  oxidation.  The  presence  of  a  high  per- 
centage of  lime  in  the  slag  is  also  essential  for  the  satisfactory 
removal  of  phosphorus,  which,  after  oxidation  to  phosphoric 
oxide  by  the  oxides  present  in  the  slag  and  dissolved  in  the 
metal,  is  fixed  by  the  lime  to  form  a  stable  phosphate.  The 
duty  then  of  the  oxidising  slag,  which  is  somewhat  similar  in 
character  to  basic  open  hearth  slag,  is  to  reduce  the  quantity 
of  all  impurities  in  the  scrap  other  than  S  to  a  required  degree, 
and  by  so  doing,  to  produce  a  bath  of  steel  as  low  as  possible 
in  C,  Mn,  Si,  and  P,  without  excessive  oxidation  of  the  steel. 

Fluxes. — The  fluxes  used  for  the  formation  of  such  a  slag 
are  in  most  cases  iron  ore  and  lime.  Limestone  is  sometimes 
used,  but,  although  initially  cheaper,  proves  less  economical 
after  the  carbon  dioxide  has  been  driven  off  at  the  expense  of 
electric  energy.  It  is  always  advisable  to  use  the  best  raw 
materials,  which  in  their  purest  forms  require  the  least  amount 
of  heat  energy  for  fusion,  and  owing  to  their  degree  of  con- 
centration have  the  maximum  power  of  chemical  action. 


METHODS   OF   MELTING   AND   EEFINING   COLD   CHAKGES      123 

Lump  ore  is  preferable,  as  in  this  form  its  action  is  more 
rapid  when  thrown  on  to  a  bath  of  steel  for  boiling  down 
purposes. 

Character  and  Appearance. — A  satisfactory  slag  will  usually 
contain  about  25-30  per  cent,  combined  FeO  and  Fe2O3,  and 
35-40  per  cent.  CaO,  the  remainder  being  MgO,  Si02,  MnO, 
etc.  A  slag  of  such  composition  may  be  recognised  by  its 
appearance  when  molten,  as  also  by  the  colour  and  fracture  of 
a  sample  removed  from  the  furnace.  The  molten  slag  should 
never  appear  glassy  and  reflect  the  light  rays  of  the  arc,  but 
should  have  a  dull  matt  appearance,  indicating  a  sufficiency  of 
lime.  A  more  positive  examination  of  the  slag  can  be  made  by 
observing  the  colour  and  fracture  of  a  cooled  sample.  Such  a 
sample  may  be  conveniently  taken  by  dipping  the  end  of  an 
iron  bar  into  the  slag  several  times  and  momentarily  with- 
drawing it,  so  that  successive  layers  may  chill  and  accumulate 
to  form  a  thick  covering  to  the  bar.  On  removal  of  the  bar 
the  slag  will  rapidly  set  and  cool,  exhibiting  a  smooth  shiny 
grey  skin,  and  cracking  off  readily.  The  fracture  should  be 
close  and  stony  in  appearance  and  have  a  grey-black  colour. 
Insufficient  oxide  is  always  evidenced  by  a  brownish  shade, 
which  is  not  always  easy  to  detect  in  artificial  light.  Infusibility, 
or  a  frothy  pastiness,  is  also  a  positive  sign  of  lack  of  oxide  in 
the  slag,  and  is  generally  accompanied  by  a  high  percentage  of 
carbon  in  the  bath  and  a  copious  production  of  pale  coloured 
smoke  and  luminous  flame.  A  sample  of  such  slag  will  be 
brown  or  yellow,  and  generally  full  of  small  cavities.  An  over- 
oxidised  slag  is  very  fluid,  black  in  colour  and  has  a  distinctly 
crystalline  fracture.  Excess  of  silica  Will  give  a  glassy  appear- 
ance to  the  slag,  either  when  molten  or  chilled,  although  the 
colour  may  be  black.  Experience  soon  enables  the  different 
characteristics  to  be  recognised  and  slag  formation  to  be 
regularly  controlled  in  each  heat  from  the  time  that  the  slag 
can  first  be  observed.  Inexperienced  melters  may  not  observe 
the  unsuitable  character  of  a  slag  until  the  bath  is  almost  ready 
for  skimming,  and  have  then  to  consider  whether  it  is  better 
to  risk  a  high  phosphorus  content  in  the  finished  steel,  or  to 
adopt  the  safer  measure  of  correcting  the  slag  composition  at 
the  expense  of  time  and  labour. 


124  THE   ELECTEO-METALLUEGY   OF   STEEL 

Formation  and  Control. — The  fluxes,  consisting  of  burnt- 
lime  and  iron  ore,  are  usually  charged  on  to  the  furnace  bottom 
before  the  scrap,  but  this  practice  is  sometimes  modified  so  that 
only  a  portion  is  at  first  charged,  the  remainder  being  added 
while  the  charge  is  melting  down.  The  proportion  of  lime 
and  ore  will  depend  upon  the  chemical  composition  of  the 
scrap,  besides  its  physical  condition  and  freedom  from  rust, 
dirt,  and  carbonaceous  matter.  Scrap  containing  much  carbon, 
either  chemically  combined  or  mechanically  mixed  in  the  form 
of  oil  or  cinders,  will  require  an  increased  addition  of  ore  before 
or  after  the  melting.  The  quantity  of  ore  required  also  de- 
pends upon  the  extent  to  which  the  scrap  is  oxidised  in  the 
furnace  either  before  or  during  melting  operations.  The 
quantity  of  ore  and  lime  used  in  a  charge  should  be  so  judged 
as  to  form  a  sufficient  slag  covering  to  the  bath ;  if  the  slag 
blanket  is  too  thin  the  arc  will  tend  to  strike  on  to  the  metal 
and  cause  a  fluctuating  load,  and  if  too  thick,  the  electrodes 
will  touch  the  slag  without  forming  a  free  arc. 

Between  two  and  a  half  and  three  and  a  half  hundredweights 
of  lime  and  ore  combined  will  usually  be  sufficient  to  form  a 
good  slag  covering  for  a  three-ton  furnace.  The  quantities  for 
smaller  and  larger  furnaces  may  be  based  on  the  ratio  of  the  re- 
spective bath  areas.  If,  however,  the  scrap  itself  contains  slag- 
forming  foreign  matter,  the  weight  of  ore  and  lime  must  be 
accordingly  reduced.  The  correct  quantity  of  ore  is  best  de- 
termined by  trial  for  a  given  class  of  scrap  and  method  of 
working,  and  it  is  better,  when  in  doubt,  to  begin  a  heat  with 
an  insufficiency  of  ore  and  to  make  additions  from  time  to  time 
until  the  character  of  the  slag  is. correct,  as  judged  by  its  appear- 
ance. The  total  weight  of  ore  used  will  serve  as  a  guide  for 
more  accurately  proportioning  the  ore  and  lime  constituents  of 
subsequent  charges.  The  proportion  of  ore  and  lime  should  be 
such  as  to  maintain  a  good  black  basic  slag  from  the  time  of 
its  first  formation  ;  in  this  way  the  elimination  of  C,  P,  Mn,  and 
Si  will  proceed  simultaneously  with  the  melting,  and  be  carried 
sufficiently  far  when  the  bath  is  hot  enough  for  skimming. 
Such  conditions  are  easily  maintained  if  the  class  of  scrap  and 
other  conditions  of  working  are  not  frequently  altered,  but  un- 
controllable circumstances,  such  as  the  fracture  of  an  electrode, 


METHODS  OF  MELTING  AND  EEFINING  COLD  CHARGES   125 

are  always  liable  to  vitiate  results  in  the  case  of  individual 
heats. 

Satisfactory  removal  of  P,  Mn,  and  Si  can  only  be  effected 
provided  the  slag  conditions  have  been  correct  for  a  certain 
length  of  time,  so  that  it  is  not  sufficient  merely  to  finish  with 
a  slag  of  proper  composition  before  its  removal  from  the  furnace. 
This  question  should  always  engage  the  careful  attention  of  the 
operator,  whose  skill  and  experience  will  enable  him  to  judge 
the  amount  of  ore  or  lime  to  add  for  correct  regulation  of  slag 
composition  during  the  process  of  melting,  and  to  estimate  the 
extent  of  P  elimination  at  a  time  when  the  bath  of  steel  is  ready 
for  skimming. 

Low  bath  and  slag  temperatures  sometimes  produce  a  mis- 
leading appearance  of  the  slag,  which  may  be  thin  and  dead- 
looking  with  apparent  indication  of  over-oxidation.  An  increase 
in  the  temperature  will  usually  suffice  to  improve  its  character, 
and  to  produce  a  slight  boil,  provided  there  is  still  sufficient 
carbon  in  the  bath. 

An  excessive  quantity  of  slag,  due  either  to  foreign  matter  in 
the  scrap  or  to  the  addition  of  large  quantities  of  ore  for  boiling 
out  carbon,  should  be  avoided  by  pouring  off  the  excess  at  inter- 
vals. A  thick  blanket  of  slag  increases  the  resistance  to  the 
passage  of  current  between  the  electrodes  and  the  metal  bath, 
and  may  at  times  prove  so  considerable  that  the  electrodes  either 
touch  the  slag  or  dip  into  it.  When  this  happens,  the  reduc- 
tion of  the  metallic  oxides  by  the  carbon  electrodes  lowers  the 
conductivity  of  the  slag,  and  further  intensifies  the  difficulty  of 
carbon  and  phosphorus  removal.  In  extreme  cases,  ,the  phe- 
nomenon of  "  pinch  effect"  may  also  develop  and  give  rise  to 
a  strongly  fluctuating  load. 

Slag  Reactions :  I.  Removal  of  Carbon. — Carbon  in  a  bath 
of  steel  is  oxidised  both  by  the  action  of  iron  or  other  metallic 
oxides  dissolved  in  it,  and  by  the  influence  of  a  covering  slag 
rich  in  either  FeO  or  Fe203,  or  these  two  combined  as  Fe3O4 ;  in 
either  case  the  speed  of  reaction  is  dependent  upon  temperature. 
A  cold  bath  of  steel  will  not  boil  even  under  a  properly  composed 
slag  unless  the  temperature  is  high  enough  to  promote  the  re- 
action, and  it  is,  therefore,  necessary  to  melt  a  charge  of  scrap  in 
such  a  manner  that  the  increasing  bath  of  steel  is  always  hot 


126  THE   ELECTRO-METALLUEGY   OF    STEEL 

enough  (except  during  feeding)  to  allow  the  elimination  of 
carbon  to  proceed.  The  best  method  of  charging  and  feeding 
cold  scrap  to  ensure  this  has  already  been  given. 

The  iron  oxide  constituent  of  the  slag  is  mainly  present  as 
FeO,  and  results  from  the  partial  reduction  of  the  higher  oxides 
derived  from  either  rust  in  the  scrap  or  added  in  the  form  of 
iron  ore  or  mill  scale.  This  partial  reduction  of  the  higher  oxides 
accompanies  the  oxidation  of  carbon  in  the  bath  of  steel  by  the 
available  oxygen,  the  reaction  being  in  this  case  exothermic. 
The  oxidation  of  carbon  by  FeO  is  endothermic,  so  that  heat 
must  be  supplied  to  the  furnace  both  to  promote  the  reaction 
and  to  maintain  the  necessary  fluidity  of  the  steel  as  its  melting- 
point  rises.  The  carbon  is  oxidised  to  carbon  monoxide,  which 
either  produces  a  frothy  condition  of  the  slag  or  bursts  through 
it  as  numerous  isolated  bubbles.  The  escape  of  carbon  monoxide 
in  this  latter  way  produces  the  appearance  of  boiling  and  the 
bath  of  steel  is  commonly  said  to  be  "  on  the  boil ". 

The  elimination  of  carbon  can  be  regularly  carried  further 
in  the  electric  furnace  than  is  either  usually  safe  or  desirable  in 
open  hearth  practice.  The  limitation  of  bath  temperature  in 
the  latter  case  is  a  question  of  heat  application,  and  there  is  thus 
grave  risk  of  the  bath  becoming  pasty  should  the  carbon  be 
removed  too  far.  The  electric  furnace,  on  the  other  hand, 
provides  a  source  of  heat,  by  which  the  temperature  of  a  bath 
can  be  raised  to  a  point  only  limited  by  the  fusing  temperature 
of  the  refractory  lining. 

II.  Eemoval  of  Phosphorus. — The  removal  of  phosphorus 
from  a  bath  of  steel  takes  place  in  two  stages.  In  the  presence 
of  oxide  of  iron,  either  dissolved  in  the  metal  or  present  in  the 
slag,  the  phosphorus  is  oxidised  to  P205,  which  may,  in  the 
absence  of  another  suitable  base,  combine  with  excess  oxide  to 
form  an  unstable  ferrous  phosphate.  This  compound  is  again 
split  up  by  metallic  iron  to  form  a  stable  iron  phosphide  FeaP, 
which  remains  in  solution  in  the  metal.  Lime  serves  as  a 
most  suitable  base  for  fixing  the  P205  originally  formed,  pro- 
vided it  is  present  in  the  slag  in  a  sufficient  degree  of  concentra- 
tion. The  combination  of  CaO  as  a  base  with  the  P205  is 
accompanied  by  evolution  of  heat,  and  therefore  results  in  the 
formation  of  a  stable  compound  which  has  the  chemical  formula 
4CaO  .  P- 


METHODS   OF   MELTING   AND   REFINING   COLD    CHARGES      127 

The  extent  to  which  phosphorus  elimination  can  be  carried 
in  arc  furnaces  is  in  great  measure  due  to  the  localisation  of 
heat,  which  by  raising  the  slag  temperature  enables  the  reac- 
tions to  proceed  to  a  far  greater  extent  than  is  possible  in  other 
furnaces.  A  slag  that  may  appear  pasty  or  thick  will,  in  the 
vicinity  of  the  arc  zones,  be  perfectly  fluid ;  this  enables  the 
composition  of  a  slag  to  be  varied  in  order  to  increase  its  power 
of  furthering  chemical  reactions. 

If  a  charge  of  scrap,  lime,  and  ore  has  been  properly  propor- 
tioned, the  carbon,  as  has  been  pointed  out,  should  at  no  time 
be  high  enough  in  the  bath,  during  the  melting  down  period,  to 
prevent  elimination  of  phosphorus  and  other  metalloids.  The 
removal  of  phosphorus  proceeds  gradually  from  the  time  a  bath 
is  first  formed,  and  should  therefore  reach  a  high  degree  of 
elimination  by  the  time  the  entire  charge  is  melted  and  hot 
enough  for  skimming ;  this  is,  of  course,  subject  to  the  main- 
tenance of  correct  slag  conditions,  which  have  been  dealt 
with. 

III.  Removal  of  Manganese  and  Silicon. — Manganese  and 
silicon,   which  may  be  present  in  the  bath  in  solution  or  as 
definite  compounds,  are  removed  by  the  oxidising  action  of  iron 
oxide  dissolved  in  the  bath  or  present  in  the  slag.     The  oxida- 
tion  proceeds   from    the   initial  stages   of  the   melting  down 
period,  and  is  influenced  by  the  amount  of  carbon  present  in 
the  bath  and  the  bath  temperature.     As  a  general  rule  there  is 
no  difficulty  in  removing  the  small  percentages  of  Mn  and  Si 
present  in  ordinary  carbon  steel  scrap  under  the  influence  of 
a  basic  oxidising  slag. 

IV.  Removal    of   Sulphur. — It   has   been   shown   (Dr.   A. 
Mueller)  that  there  is  a  removal  of  sulphur  during  the  refining 
operation  under  a   basic   oxidising  slag.     The  reaction  which 
may  be  expressed  by  the  equation — 

FeS  +  2FeO  =  3Fe  +  SO2, 

can,  however,  only  proceed  when  the  FeS  +  FeO  are  present  in 
a  sufficient  degree  of  mutual  concentration. 

The  figures  in  the  table  below  (Mueller)  show  the  degree  of 
sulphur  removal  from  three  charges  of  liquid  steel  during  the 
oxidising  and  dephosphorising  operation  : — 


128 


THE    ELECTRO-METALLURGY    OF    STEEL 


C. 

Mn. 

P. 

S. 

Si 

Per  Cent.  S 
Removal. 

Liquid  steel  charged     . 

•19 

•76 

•02 

•045 

trace 

After  ore  addition 

•13 

•48 

•01 

•044 

Before  slagging    . 

•09 

•30 

trace 

•033 

26-7  per  cent. 

Liquid  steel  charged     . 

•2 

•64 

•03 

•056 

After  ore  addition 

•2 

•52 

•02 

•056 

Before  slagging    . 

•12 

•28 

trace 

•039 

30-4  per  cent. 

Liquid  steel  charged     . 

•23 

•62 

•03 

•059 

After  ore  addition 

•22 

•44 

•01 

•048 

•21 

•42 

•01 

•045 

Before  slagging    . 

•21 

•38 

•005 
, 

•045 

23  -7  per  cent. 

Conditions  of  Bath  before  Skimming. — From  a  chemical 
standpoint  sufficient  has  been  said,  both  in  regard  to  slag  forma- 
tion and  bath  reactions,  to  show  that  the  process  up  to  the 
time  of  skimming  is  conducive  to  the  maximum  removal 
of  P,  Mn,  and  Si  with  reduction  of  the  carbon  to  a  low 
figure. 

Carbon. — The  carbon  content  of  the  bath  should  be  about 
'06  per  cent,  to  '08  per  cent,  and  if  the  slag  composition  and 
temperature  are  correct,  may  be  easily  judged  by  the  very  slight 
boil  which  will  still  be  exhibited.  Further  removal  of  carbon, 
accompanied  by  excessive  over-oxidation,  is  likely  to  cause 
trouble  in  the  subsequent  deoxidation  and  desulphurising  treat- 
ments, and  will  also  cause  irregularity  of  carbon  absorption,  if 
carburising  with  anthracite  or  other  form  of  carbon  is  to  follow 
the  skimming  operation.  A  spoon  sample  is  usually  taken  and 
poured  on  to  an  iron  plate  to  form  a  thin  narrow  strip,  which 
should  bend  double  without  fracture  after  being  quenched  in 
water  if  the  carbon  is  sufficiently  low.  A  small  piece  of 
aluminium  thrown  into  the  spoon  before  pouring  will  produce 
a  strip  of  sound  steel  free  from  blow  holes  ;  powdered  ferro- 
silicon  is  not  satisfactory  for  this  purpose  as  an  excess  is  likely 
to  cause  brittleness,  and  so  vitiate  the  test. 

Phosphorus. — Phosphorus  should  not  exceed  about  '012  per 
cent,  in  the  bath  for  tool  and  special  alloy  steels ;  this  will  be 
usually  equivalent  to  a  final  percentage  of  :018  per  cent,  to  '02  per 
cent,  in  the  finished  steel.  It  is  difficult  to  explain  the  dis- 
crepancy, since  the  increase  cannot  possibly  be  due  to  a 


METHODS   OF   MELTING   AND    KEFINING    COLD    CHAEGES      129 

reduction  of  phosphorus  from  the  inappreciable  quantity  of 
the  oxidising  slag  left  behind  after  a  careful  skimming.  For 
castings  there  is  seldom  any  necessity  for  reducing  the  phos- 
phorus below  "03  per  cent,  in  the  finished  steel. 

After  working  through  a  few  trial  heats  with  a  given  quality 
of  scrap  and  correct  slag  conditions,  experience  will  show 
whether  the  phosphorus  is  sufficiently  low  when  the  bath  is 
otherwise  ready  for  skimming.  If  there  is  any  doubt,  a  bath 
sample  may  be  taken  at  least  half-an-hour  before  it  is  expected 
to  skim  the  bath,  so  that  no  delay  will  be  occasioned  pending  the 
result.  A  charge  composed  of  ordinary  steel  scrap  will  usually 
be  low  enough  in  phosphorus  when  entirely  melted  and  ready 
for  skimming.  Should,  however,  the  phosphorus  be  too  high 
after  melting  down  a  charge,  it  is  often  best  to  pour  off  the  bulk  of 
the  slag  and  add  fresh  lime  and  ore  to  effect  the  final  elimination. 

Manganese. — Manganese  in  the  bath  may  vary  from  a  trace 
up  to  0' 4  per  cent,  according  to  the  carbon  and  manganese  content 
of  the  steel  scrap.  It  is  preferable  that  the  manganese  be  0'15 
per  cent,  or  under,  although  a  higher  figure  is  quite  permissible, 
provided  it  is  not  the  result  of  improper  slag  conditions.  It  is 
often  an  advantage  to  be  obliged  to  add  0*3  per  cent.  Mn  before 
casting  in  the  form  of  either  spiegel  or  ferro -manganese,  as  this 
generally  enables  a  final  adjustment  of  the  carbon  content  of 
the  steel  to  be  made  before  pouring  without  recourse  to  white 
iron  or  haematite  pig.  Scrap  of  medium  carbon  and  O8  per  cent. 
to  1  per  cent.  Mn  will  usually  melt  down  to  a  bath  carrying 
0'3  per  cent,  to  0'4  per  cent.  Mn,  unless  the  slag  is  carefully 
watched  and  kept  up  to  a  proper  degree  of  oxidation.  It  does 
not  always  follow  that  a  percentage  of  manganese  in  the  bath 
above  0'2  per  cent,  signifies  also  a  high  phosphorus  content  due 
to  insufficient  oxidation,  but,  generally  speaking,  the  possibility 
is  greater,  and  caution  should  be  exercised. 

When  using  scrap  that  leaves  over  0'15  per  cent.  Mn  in  the 
bath,  it  is  advisabls  to  determine  the  manganese  in  a  bath  sample 
taken  at  a  later  period,  especially  where  the  manganese  has 
to  be  within  close  limits  in  the  final  specification. 

Silicon. — If  carbon  and  phosphorus  are  properly  eliminated 
as  indicated  above,  the  silicon  will  invariably  be  reduced  to 
a  trace.  An  unexpectedly  high  percentage  of  silicon  in  the 

9 


130  THE    ELECTRO-METALLURGY   OF   STEEL 

finished  steel  is  sometimes  accompanied  by  high  manganese  and 
phosphorus  contents,  which  indicate  improper  slag  conditions 
before  skimming. 

Temperature. — Temperature  is  an  important  factor  in  skim- 
ming operations,  and  should  always  be  carefully  observed 
immediately  before  skimming.  Lack  of  heat  often  renders  the 
slag  thin  and  difficult  to  gather,  and'  will  also  cause  the  steel  to 
set  on  the  skimming  tools  and  slagging  spout.  A  bath  that  has 
not  a  sufficient  reserve  of  heat  to  resist  scumming  over  after 
removal  of  slag  will,  if  carburising  is  to  follow,  result  in  a  low 
and  variable  absorption  of  carbon.  This  makes  the  subsequent 
operation  of  finishing  more  difficult  and  uneconomical,  especially 
when  further  carbon  has  to  be  added  in  the  form  of  high  grade 
white  iron  or  haematite  pig.  The  chemical  and  physical  con- 
ditions of  the  bath  should  be  as  nearly  as  possible  the  same  for 
successive  heats,  this  being  one  of  the  chief  factors  which  lead 
to  economy  of  production  and  uniformity  of  the  chemical  and 
physical  properties  of  the  product. 

Removal  of  Oxidising  Slag. — If  the  correct  slag  and  tempera- 
ture conditions  are  fulfilled,  the  slag  can  be  removed  without 
any  difficulty.  The  furnace  is  tilted,  and  the  bulk  of  slag  poured 
off  until  the  arcs  become  broken  and  snappy.  The  main 
switch  is  then  opened,  and  the  electrodes  raised  to  allow  free 
movement  of  the  skimming  tools  over  all  parts  of  the  bath ; 
neglect  to  do  this  often  leads  to  a  broken  electrode — particularly 
in  the  case  of  graphite  electrodes  of  small  diameter — with  the 
probable  result  of  unknown  absorption  of  carbon  by  the  bath. 
The  remainder  of  the  slag  is  then  gently  skimmed  off,  the  last 
remnant  being  more  easily  removed  after  thickening  with  a  few 
shovels  of  fine  dolomite  or  lime.  Care  is  taken  throughout 
the  operation  to  prevent  an  accumulation  of  slag  on  the  lip  of 
the  spout,  which  makes  further  removal  very  difficult. 

Skimming  appears  to  be  a  simple  operation,  but  if  properly 
done,  requires  considerable  skill.  Experience  alone  will  enable 
a  man  to  skim  in  the  shortest  time  without  loss  of  metal  and  to 
the  necessary  degree  of  removal,  which  is  a  matter  of  judgment 
according  to  the  operations  that  follow.  After  skimming  has 
been  completed,  the  furnace  is  again  tiltecl  to  th§  normal  position 
and  is  ready  for  the  next  operation. 


METHODS    OF   MKLTING   AND   EEFINING   COLD   CHARGES      131 

Carburising. — Carburising  is  best  done  by  adding  to  the 
skimmed  bath  a  calculated  quantity  of  anthracite,  or  other  form 
of  carbon  low  in  ash  and  volatile  matter.  The  material  chosen 
should  contain  a  maximum  percentage  of  carbon,  and  be  in  a 
physical  condition  conducive  to  rapid  absorption  by  the  steel. 
A  high  percentage  of  ash  will  form  a  slag  covering,  which 
mechanically  prevents  absorption  of  the  last  additions  made ;  this 
applies  particularly  to  high  carbon  steels,  and  to  furnaces  in 
which  the  bath  area  bears  a  small  ratio  to  its  volume.  A  high 
percentage  of  volatile  matter  (10  per  cent,  and  over)  in  anthracite 
is  equally  objectionable,  as  it  causes  the  small  particles  to  cake 
together  as  soon  as  they  touch  the  bath,  which  prevents  proper 
contact  and  mixing. 

If  the  conditions  of  the  bath  are  correct  before  skimming 
and  this  latter  operation  is  properly  carried  out,  the  addition  of 
carbon  will  be  accompanied  by  a  boil,  which  is  sometimes  con- 
siderable at  first  but  gradually  subsides  as  the  last  additions  are 
made.  This  reaction,  at  the  beginning,  must  be  carefully 
watched  and  kept  under  control  by  adding  only  small  quantities 
until  the  period  of  violent  ebullition  is  passed ;  the  carbon  may  then 
be  added  more  rapidly  without  fear  of  a  boil-out.  This  ebullition 
is  particularly  useful  as  it  serves .  to  keep  the  carbon  and  steel 
continually  in  movement.  When  the  practice  of  adding  ferro- 
silicon  is  adopted,  it  is  preferred  by  many  to  destroy  this  effect 
by  making  the  addition  before,  instead  of  after,  the  carbon  is 
added  ;  this  method  may  be  followed  quite  satisfactorily,  pro- 
vided only  a  small  addition  of  carbon  is  to  be  made,  but  in  the 
case  of  high  carbon  steels  the  slag  formed  by  the  ferro-silicon 
addition,  together  with  the  lack  of  mechanical  mixing,  make 
it  far  more  difficult  to  effect  a  regular  and  maximum  degree 
of  carbon  absorption.  The  violent  boil  after  the  first  carbon 
addition  does  not  take  place  immediately,  but  only  proceeds 
after  the  carbon  has  reached  a  certain  degree  of  concentra- 
tion in  the  bath,  when  it  then  reacts  with  the  dissolved  oxides 
present.  The  violence  of  the  reaction  falls  off  as  the  reduction 
of  oxides  becomes  more  complete.  With  increasing  weights  of 
carbon  added  to  the  bath,  the  proportion  actually  absorbed  will 
similarly  increase ;  this  is  obvious,  since  the  first  addition  made 
may  be  regarded  as  lost  during  the  oxide  reaction.  The  ratio 


132  THE   ELECTRO-METALLURGY   OF    STEEL 

of  the  actual  amount  of  carbon  absorbed  to  the  weight  of 
carbon  added  is  conveniently,  but  perhaps  incorrectly,  expressed 
in  terms  of  percentage  efficiency,  so  that  for  small  additions 
the  efficiency  will  be  lower  than  for  large.  The  "  efficiency  "  will 
vary  between  40  per  cent,  to  55  per  cent.,  and  will  be  lower 
still  for  very  small  additions.  The  conditions  that  favour 
maximum  absorption  are  : — 

(a)  good  bath  temperature, 

(6)  freedom  of  bath  from  slag,    - 

(c)  steel  not  too  over-oxidised, 

(d)  low  ash  and  volatile  content  of  the  carburiser  employed. 

(e)  physical  state  of  the  carburiser,  such  as  to  prevent  loss 
by  dusting  and  to  offer  a  maximum  surface,  of  contact. 

Anthracite  of  good  quality  is  usually  employed,  and  should 
be  carefully  sized  by  rejecting  all  particles  passing  through  a 
30  mesh  sieve,  together  with  the  oversize  from  a  quarter- inch 
or,  better  still,  a  T3^  inch  mesh  sieve.  Certain  varieties  of  Welsh 
anthracite  are  very  suitable,  an  excellent  sample  of  which  has 
the  following  analysis  : — 

Fixed  carbon 91 '6  per  cent. 

Volatile  matter  .  .  .  .  5'3  ,, 
Sulphur  and  phosphorus  .  .  0'8  ,, 
Ash about  1*75  ,, 

The  ash  and  volatile  matter  are  usually  rather  higher,  rising 
up  to  5  per  cent,  and  7  per  cent,  respectively.  Sulphur  and 
phosphorus  should  be  as  low  as  possible.  Anthracite  should  be 
stored  in  a  dry  place  as,  if  exposed  to  weather,  it  will  carry 
sufficient  moisture  to  considerably  vitiate  the  calculated  weight. 
After  the  last  addition  of  carbon  has  been  made,  it  is  advisable 
to  wait  until  all  action  has  ceased,  as  shown  by  the  entire  drop 
of  flame  from  the  furnace,  before  giving  a  final  stir  to  free  any 
carbon  held  together  by  slag.  The  bath  is  then  ready  for  the 
next  and  final  stage  of  the  process,  conducted  under  a  power- 
fully reducing  slag. 

Basic  Reducing  Slag. — Function. — Up  to  the  time  of  car- 
burising  the  process  is  usually  conducted  under  conditions  that 
exert  an  oxidising  action  on  the  bath  of  steel.  The  act  of 
carburising  undoubtedly  removes  a  portion  of  the  iron  oxide 


METHODS   OF  MELTING   AND   REFINING   COLD   CHARGES      133 

dissolved,  but  is  incapable  of  carrying  the  reduction  past  that 
point  at  which  there  is  a  chemical  equilibrium  between  the 
carbon  and  oxide  still  remaining  dissolved  in  the  steel.  It  re- 
quires, then,  a  far  more  powerful  reducing  agent  to  deoxidise  the 
steel  to  such  an  extent  that  it  will  produce  a  perfectly  sound 
ingot.  For  this  purpose  a  strongly  reducing  slag  composed  of 
lime,  fluor  spar  and  carbon  is  used,  and  this  is  frequently  supple- 
mented by  ferro-silicon  added  to  the  bath  after  skimming  and 
in  very  limited  quantities  during  the  process  of  slag  deoxidation. 
Deoxidation  is,  however,  possible  without  the  use  of  ferro-silicon, 
but  the  period  of  refining  is  more  prolonged. 

Another  important  function  performed  by  this  slag  is  the 
removal  of  sulphur,  which  can  be  carried  to  a  considerable 
degree.  The  slag  also  serves  as  a  perfectly  neutral  covering  to 
the  bath,  so  that  the  latter  can  be  held  at  any  desired  tem- 
perature without  changing  its  chemical  composition  ;  this  is 
exceedingly  useful,  as  it  enables  the  steel  to  be  held  in  a  tranquil, 
inert  condition  pending  analysis  of  bath  samples,  or  in  the 
event  of  shop  delays.  These  conditions  are  also  conducive  to 
the  elimination  of  slag  suspended  in  the  bath  as  minute  particles. 
The  slag,  when  of  correct  composition,  will  not  contain  more 
than  0'5  per  cent,  of  metallic  oxides,  so  that  ferro-alloy  additions 
may  be  made  according  to  calculation  without  any  allowance 
for  oxidation  loss. 

Fluxes. — (a)  Ferro-silicon. — Although  ferro-silicon  cannot  be 
strictly  regarded  as  a  flux  it  is  convenient  to  consider  it  under 
this  category,  as,  apart  from  the  part  it  plays  in  the  deoxidation 
of  the  bath,  it  supplies  at  the  same  time  a  large  part  of  the 
silica  content  of  the  reducing  slag.  The  greater  part  of  the 
total  ferro-silicon  addition  is  made  to  the  bath  of  steel  im- 
mediately after  carburising.  A  rich  alloy,  containing  at  least 
45  per  cent.  Si,  should  be  used  for  this  purpose,  otherwise  the 
chilling  effect  of  the  larger  quantities  required  for  a  poorer 
grade  will  prevent  proper  absorption  by  the  already  cooled  bath. 
The  alloy  should  be  crushed  small  to  expose  as  large  a  surface 
as  possible  to  the  steel  and  so  hasten  absorption. 

For  use  after  the  reducing  slag  is  formed,  the  ferro-silicon 
should  not  contain  more  than  45  per  cent,  to  50  per  cent.  Si, 
otherwise  its  specific  gravity  will  be  so  low  that  the  alloy  will 


134  THE   ELECTEO-METALLUEGY  OF   STEEL 

only  with  difficulty  pass  through  the  slag  and  enter  the  steel. 
These  latter  additions  of  the  alloy  should  also  be  made  in  lump 
form,  avoiding  small  pieces  which  might  be  held  up  in  the 
slag.  The  most  convenient  grade  is  undoubtedly  the  45  per 
cent,  to  50  per  cent.  Si  alloy,  as  it  serves  equally  well  for  both 
purposes  ;  the  dust  and  smaller  pieces  are  added  to  the  naked 
bath,  while  the  lumps  are  reserved  for  later  use. 

(b)  Lime. — Hard-burnt  lime  is  most   commonly  used,   al- 
though crushed  limestone  is  occasionally  employed.      In  the 
latter  case  the  same  objections  arise  as  when  it  is  used  for  the 
oxidising  slag  formation ;  also,  the  formation  of  the  reducing 
slag  is  delayed  until  all  the  carbon  dioxide  has  been  driven  off. 
The   lime  should   be  used   in  small  pieces,  or  broken  up  just 
before   use  by  moistening  with  water;  this  is   important,    as 
fusion  of  the  fluxes  and  formation  of  the  slag  will  otherwise  be 
delayed,  more   especially  if   the  bath  has  been   well-skimmed 
before  carburising.     It  is  advisable  to  select  a  lime  that  is  not 
too  high  in  sulphur  or  phosphorus,  but  apart  from  this  almost 
any  burnt  lime  is  suitable. 

(c)  Fluor  Spar. — Fluor  spar  is  a  most  useful  flux  both  for 
promoting  slag   formation   and  for  adjusting  the  fluidity  and 
general  character  of  the  slag  once  formed.     Its  action  is  entirely 
mechanical,  and  only  serves  to  diminish  the  stiffness  of  a  limey 
and  almost  mono-basic  slag  by  its  extreme  fluidity  when  molten. 
Its  action  is  powerful,  and  so  it  is  only  used  sparingly  when 
rectifying   the    character   of   the   slag    after   initial   formation. 
Fluor  spar  is   often  associated  with  galena  (PbS)  and  should 
be  examined  for  this  latter  mineral,  which,  if  present  in  any 
quantity,  will  render  the  spar  unfit  for  use.     Iron  has  a  greater 
affinity  for  sulphur  than  lead  has,  so  that  simple  replacement 
will  follow  direct  contact  with  the  bath. 

(d)  Silica. — Sand  is  sometimes  employed  as  a  substitute  for 
fluor  spar,  either  wholly  or  in  part,  but  is  liable  to  produce  a 
slag  too  rich  in  silica,  which  is  usually  in  the  neighbourhood  of 
25  per  cent,  when  ferro-silicon  is  used  for  deoxidising  the  bath. 

(e)  Carbon  Dust. — This  is   added  to  the  slag  for  reducing 
the  metallic  oxides  which  are  present,  and  for  the  subsequent 
formation  of  calcium  carbide.     Anthracite,  electrode  carbon,  or 
petroleum  coke  is  used  for  this  purpose  in  the  form  of  fine  dust, 


METHODS   OF   MELTING  AND  REFINING   COLD   CHARGES      135 

which  reduces  the  risk  of  carbon  passing  into  the  steel  and 
effects  reduction  in  the  minimum  of  time. 

Character  and  Appearance. — The  physical  characteristics 
of  a  proper  finishing  slag  are  very  well  defined,  and  undergo 
marked  and  rapid  changes  with  slight  variation  in  its  con- 
dition ;  it  is,  therefore,  an  easy  matter  to  adjust  the  com- 
position and  degree  of  fluidity  from  time  to  time,  so  as  to 
satisfy  the  conditions  most  favourable  to  deoxidation  and  de- 
sulphurising. 

The  slag  first  formed  by  fusion  of  the  lime  and  spar,  and 
on  to  which  a  quantity  of  carbon  dust  has  been  thrown,  will  at 
first  be  brown  or  brown-yellow  in  colour  and  rather  stiff;  on 
rise  of  temperature  it  will  become  much  thinner,  and,  if  there 
is  sufficient  carbon  present  for  reduction  of  oxides,  the  colour 
will  change  to  a  pale  yellow,  and  subsequently  to  white. 
During  this  change,  a  pronounced  reaction  takes  place  in  the 
slag,  giving  it  a  frothy  appearance  due  to  the  evolution  of 
carbon  monoxide.  The  surface  should  at  all  times  appear 
quite  dull  if  not  frothing,  a  glassy  appearance  indicating 
presence  of  metallic  oxides  in  the  slag,  or  lack  of  basicity.  Ex- 
perience alone  can  indicate  to  which  cause  the  faulty  character 
is  due.  As  a  general  rule  the  slag  should  have  a  creamy  con- 
sistency, be  white  or  greyish  in  colour,  fall  to  a  fine  powder  on 
cooling  from  redness,  and,  when  moistened  with  water,  exhibit 
the  presence  of  free  calcium  carbide  by  the  smell  of  evolved 
acetylene.  The  peculiar  property  which  the  slag  possesses  of 
falling  to  a  fine  powder  is  not  necessarily  an  indication  of  the 
presence  of  calcium  carbide,  since  the  slag  will  often  fall  at 
almost  a  red  heat  before  any  decomposition  of  CaC2  is  possible. 
It  is  rather  an  indication  of  a  high  lime  content  and  freedom 
from  metallic  oxides. 

Analyses  of  Reducing  Slags  : — 

CaC2  2-1  9-77  5-74  4-59 

CaF2  20-7  20-0  22-6  20-5 

CaS  -74  -51  -73  -47 

CaO  53-0  46-7  50-1  50-8 

SiO2  14-17  16-21  14-26  15-95 

A1.2O3  3-18  4-69  3-95  3-96 

MgO  2-95  1-99  3-65  2-95 

Free  coke  -80  '66  1-24  1-32 


136  THE    ELECTRO-METALLURGY   OF    STEEL 

The  above  are  analyses  of  slags  taken  from  a  15-ton  furnace 
used  for  refining  liquid  steel,  but  usually  the  SiO2  is  somewhat 
higher,  being  about  20  per  cent,  or  more  when  ferro- silicon  is 
used  to  effect  preliminary  deoxidation  of  the  bath.  With  such 
strongly  reducing  slags  almost  the  entire  deoxidation  can  be 
effected  by  the  calcium  carbide  in  the  slag  without  the  use  of 
ferro-silicon,  provided  the  bath  is  not  in  a  highly  oxidised  con- 
dition. Slag  samples  rich  in  calcium  carbide  usually  have 
a  pale  grey  colour  and  smooth  skin,  and  fall  to  a  greyish-white 
powder. 

Slag  Formation  and  Control  of  Bath  Deoxidation. — The 
addition  of  ferro-silicon  immediately  following  carburisation  of 
the  bath  has  already  been  mentioned.  When  this  practice  is 
followed,  it  is  usually  quite  safe  to  add  a  quantity  equivalent  to 
0'2  per  cent.  Si  in  the  bath  without  any  risk  of  unoxidised  Si 
remaining  in  the  steel.  This  applies  to  charges  that  have  been 
melted  and  worked  down  normally  from  a  fairly  low  carbon 
scrap,  yielding  an  oxidised  bath  at  the  time  of  skimming. 
After  the  ferro-silicon  has  all  worked  through,  which  may  be 
aided  by  rabbling,  the  fluor  spar  is  added  together  with  about 
half  the  lime,  this  being  done  to  promote  more  rapid  fusion  and 
formation  of  a  slag  covering.  After  partial  fusion  of  the  fluxes 
added,  a  liberal  quantity  of  carbon  dust  is  thrown  on  and 
allowed  to  work  through  for  a  few  minutes.  A  slag  sample  is 
then  taken,  and  if  found  to  be  pale  brown  or  yellow,  a  bath 
sample  can  be  taken  for  the  analysis  of  carbon,  manganese,  and 
other  constituents,  after  well  stirring  the  bath.  However  per- 
fect the  slag  may  be,  this  sample  should  on  no  account  be  taken 
unless  the  bath  is  fairly  hot,  but  well  below  the  temperature  of 
casting,  otherwise  the  sample  cannot  be  accepted  as  truly 
representative  owing  to  the  non-homogeneity  of  the  bath.  By 
retaining  one-half  of  the  lime  from  the  first  addition  of  fluxes, 
time  is  saved  by  the  more  rapid  formation  of  a  slag  covering, 
which,  when  well  fused,  enables  a  spoon  sample  to  be  taken 
without  risk  of  spoiling  the  spoon.  The  remainder  of  the  lime 
is  added  as  soon  as  the  bath  sample  has  been  taken,  the  in- 
creasing temperature  of  the  slag  being  sufficient  to  maintain 
the  necessary  degree  of  fluidity.  The  slag  should  be  carefully 
and  frequently  observed  by  taking  a  sample  on  an  iron  bar. 


METHODS   OF   MELTING  A^B  BEFINING   COLD   CHAKGES      137 

Any  tendency  to  revert  to  a  darker  shade  of  yellow  or  brown 
demands  an  addition  of  carbon  dust ;  if  too  thick,  a  small 
quantity  of  fluor  spar  should  be  given,  and  if  too  thin,  lime  is 
required.  The  change  in  the  appearance  of  the  slag  is  accom- 
panied by  a  simultaneous  change  in  the  appearance  of  the 
smoke  issuing  from  the  furnace  doors  or  roof.  After  the  first 
addition  of  fluxes,  a  copious  quantity  of  a  pale  grey-yellow 
smoke  will  be  evolved  together  with  much  luminous  flame  ;  as 
slag  formation  proceeds  and  the  oxides  become  reduced,  the 
smoke  will  diminish  in  volume  and  become  whiter  and  less 
dense,  finally  assuming  the  appearance  of  a  thick,  white  haze. 
The  flame  also  will  subside  and  become  less  luminous.  Carbon 
dust  may  be  used  liberally  without  fear  of  carbon  entering  the 
steel,  and  is,  moreover,  essential  to  the  formation  of  free 
calcium  carbide.  The  total  quantity  of  fluxes  used  should 
be  sufficient  to  form  a  good  covering  to  the  bath,  which  will 
prevent  carbon  absorption  by  the  steel  from  the  carbon  dust 
added. 

The  process  of  deoxidation  of  the  bath  by  slag  reaction  is  a 
function  of  time  dependent  upon  the  degree  of  oxidation,  slag 
composition,  and  temperature,  and  is  more  prolonged  than  when 
aided  by  other  deoxidisers.  Whether  ferro-silicon  is  used  or  not 
for  effecting  a  preliminary  and  partial  deoxidation  after  skim- 
ming, it  is  a  common  practice  to  make  small  additions  of  this 
alloy,  equivalent  to  '05  per  cent,  to  1  per  cent.  Si,  at  a  later  stage 
to  assist  deoxidation  by  the  carbide  bearing  slag.  These  small 
additions  may  be  made  from  time  to  time  according  to  the  con- 
dition of  the  steel,  as  judged  by  the  degree  of  "  wildness  "  shown 
on  solidification  of  a  spoon  sample  poured  into  a  small  mould. 
When  ferro-silicon  is  used  purely  as  a  deoxidising  agent,  the  Si 
is  oxidised  and  unites  with  the  unreduced  metallic  oxides  still 
present  to  form  silicates.  The  greater  part  of  these  silicates 
certainly  passes  from  the  steel  into  the  slag,  but  the  remaining 
portion  will  exist  either  in  a  state  of  fine  suspension  or  true 
solution.  In  either  case  the  silicates  which  remain  in  the  steel 
at  the  time  of  pouring  will  segregate  on  solidification  and  be 
distinguishable  under  microscopic  examination.  For  this 
reason,  when  making  certain  special  classes  of  steel,  it  is  pre- 
ferable to  use  ferro-silicon  and  other  slag  forming  deoxidisers  as 


138  THE   ELECTRO-METALLURGY   OF   STEEL 

sparingly  as  possible  and  to  utilise  rather  the  deoxidising  powers 
of  the  calcium  carbide  bearing  slag  to  its  utmost  extent. 

Slag  Reactions. — (a)  Removal  of  Oxygen. — The  removal  of 
dissolved  iron  oxide  from  the  bath  results  from  the  action  of 
ferro-silicon,  when  used,  and  of  the  calcium  carbide  formed  in  the 
slag.  In  the  case  of  the  former  the  reaction  takes  place  between 
the  Si  and  0  in  the  bath  itself,  whereas  in  the  latter  case  it  is 
purely  a  contact  reaction.  The  reducing  action  of  ferro-silicon 
is  simply  due  to  the  fact  that  Si  has  a  greater  affinity  for  O  than 
Fe,  the  reaction  taking  place  with  evolution  of  heat.  With  re- 
gard to  the  true  slag  reaction,  the  power  of  deoxidation  is  due 
to  formation  of  calcium  carbide,  which,  when  in  contact  with  a 
bath  containing  dissolved  oxides,  is  immediately  decomposed 
with  liberation  of  CO  and  CaO,  as  may  be  expressed  by  the 
equations  :— 

(1)  4CaC2  +  3Fe3O4  =  9Fe  +  SCO  +  4CaO, 

(2)  CaC2  +  3FeO  =  3Fe  +  2CO  +  CaO. 

The  reaction  is  accompanied  by  flame  resulting  from  the 
combustion  of  the  CO  liberated.  Owing  to  the  liberation  of 
carbon  monoxide  the  amount  of  flame  issuing  from  the  furnace 
becomes  insignificant  when  the  slag  is  white  and  the  steel 
freed  from  oxide,  and  this  is  also  a  sure  indication  of  the  satis- 
factory completion  of  the  reaction.  The  elimination  of  oxygen 
solely  by  means  of  the  carbide  reaction  is  necessarily  slower 
than  when  aided  by  ferro-silicon. 

(b)  Removal  of  •Sulphur. — Sulphur  can  be  eliminated  to  a 
remarkable  degree  by  the  action  of  a  finishing  slag  having  the 
characteristics  previously  described.  It  has  been  shown  that 
the  removal  of  sulphur  is  comparatively  slow  until  the  slag  is 
free  from  oxides,  as  indicated  by  its  white  appearance  or  the 
presence  of  calcium  carbide  ;  at  this  point  the  sulphur  reaction 
proceeds  with  great  rapidity,  and  its  elimination  is  soon  com- 
plete. This  is  clearly  demonstrated  by  the  analyses  of  slag 
samples  taken  at  different  periods  during  the  reducing  stage 
(Dr.  A.  Mueller)  :— 


• 


METHODS   OF   MELTING   AND   EEFINING   COLD   CHARGES      139 


Colour. 

FeO  and  Fe203 

MuO. 

S. 

After  carburising 
,,     final  flux  addition 
Before  tapping   . 

Greyish-  brown 
White  granular 
„      powder 

1-25  per  cent. 
•27    „       „ 
•13    „       „ 

1-44 
•52 

trace 

•39 
•9 
1-22 

There  is  no  difficulty  in  reducing  the  sulphur  in  the  steel  to 
below  "02  per  cent.,  the  sulphur  being  fixed  in  the  slag  as  CaS, 
which  can  only  exist  as  such  in  the  absence  of  manganese  and 
iron  oxides.  This  accounts  for  the  negligible  reduction  of 
S  before  the  slag  becomes  white.  The  reactions  which  take 
place  may  be  represented  by  the  following  equations,  into  each 
of  which  carbon,  either  free  or  combined,  enters  (Amberg)  :— 

(1)  FeS  +  CaO  +  C  =  Fe  +  CaS  +  CO. 

(2)  3FeS  +  2CaO  +  CaC2  =  3Fe  +  3CaS  +  2CO. 

The  removal  of  sulphur,  according  to  the  first  equation,  is 
probably  what  also  takes  place  in  a  blast  furnace,  where  the 
production  of  low  sulphur  pig-iron  is  favoured  by  a  somewhat 
basic  slag  and  a  high  furnace  temperature.  There  seems  little 
doubt,  therefore,  that  the  first  equation  represents  the  correct 
nature  of  the  chemical  reaction.  At  the  higher  temperatures 
of  the  arc  zone,  where  CaC2  is  formed,  the  reaction  may  take  the 
form  of  the  second  equation.  In  the  same  way,  silicon  dust  or 
other  reducing  agent  present  in  the  slag  will  suffice  to  prevent 
the  reversible  reaction  according  to  the  equation  : — 
FeS  +  CaO  ^  CaS  +  FeO. 

Alloy  Additions. — The  final  addition  of  ferro-manganese  and 
ferro-silicon  for  specification  purposes  should  not  be  made  until 
after  the  steel  has  been  "  killed  "  by  slag  reaction  alone,  or  with 
the  aid  of  small  ferro-silicon  additions,  and  then  not  less  than 
five  minutes  before  actually  casting.  When  ferro-silicon  has 
been  used  to  assist  deoxidation,  it  is  usual  to  base  the  calcu- 
lation of  the  final  addition  of  this  alloy,  for  specification  pur- 
poses, upon  a  bath  content  of  01  per  cent.  Si.  Nickel,  ferro- 
chrome,  and  other  alloys  should  be  added  before  the  manganese 
and  silicon,  and  the  bath  rabbled  to  ensure  thorough  mixing  and 
homogeneity.  When  large  additions  are  to  be  made  the  alloys 
should  be  added  in  small  quantities  at  a  time,  and  it  should  be 
ascertained  that  the  bottom  is  perfectly  clear  by  feeling  it  with 


140  THE   ELECTEO-METALLURGY   OF   STEEL 

an  iron  bar  before  each  addition  is  made.  By  taking  this 
simple  precaution  there  is  never  the  slightest  risk  of  the  bath 
chilling  and  setting  on  the  bottom  of  furnaces  in  which  no 
hearth  heating  is  developed.  Aluminium,  if  used  at  all,  should 
not  exceed  4  oz.  to  the  ton  when  making  ingots,  but  for  foundry 
work  it  is  often  advisable  to  increase  this  proportion  consider- 
ably as  a  precautionary  measure  against  subsequent  oxidation, 
especially  in  the  case  of  green  sand  casting. 

Temperature  Control. — Temperature  control  is  an  impor- 
tant factor  in  refining  and  finishing  electric  steel.  After  skim- 
ming and  carburising,  the  bath  temperature  will  be  very  low, 
and  it  is  advisable,  on  again  heating,  to  operate  at  a  load  that 
will'fuse  the  fluxes  added  and  raise  the  bath  temperature  to  a 
degree  suitable  for  spoon  sampling  at  the  end  of  about  15  to 
20  minutes.  The  ratio  of  this  load  to  full  load  will  be  found  by 
experience,  and  will  vary  according  to  the  furnace  capacity  and 
the  temperature  of  the  steel  after  carburising.  The  temperature 
of  the  bath  when  sampling  should  be  moderate,  but  not  hot 
enough  for  casting,  and  should  then  be  held  with  little  further 
rise  until  the  steel  is  nearly  ready  for  casting,  or  for  receiving 
large  additions  of  ferro-alloys.  In  the  case  of  furnaces  of  small 
capacity — 30  cwts.  or  less — it  is  by  no  means  easy  to  so  regulate 
the  temperature,  as  a  small  margin  of  power  over  and  above 
that  equivalent  to  the  constant  radiation  loss  will  suffice  to  raise 
the  temperature  rapidly.  When  small  additions  of  ferro-silicon 
are  made  during  the  process  of  deoxidation,  it  is  of  the  greatest 
importance  that  the  bath  temperature  should  not  exceed,  but 
preferably  remain  below,  normal  casting  temperature,  otherwise 
difficulty  may  be  encountered  in  "  killing"  the  steel,  and  high 
silicon  contents  result. 

Various  methods  of  judging  temperature  are  employed. 
Some  merely  take  a  sample  in  a  spoon  about  3  inches  in  diameter, 
allow  it  to  remain  a  few  seconds,  and  then  upset  the  steel  on  to 
an  iron  plate  ;  if  the  steel  runs  freely,  and  leaves  the  spoon  per- 
fectly clear,  it  is  deemed  hot  enough  for  casting.  Another 
method  commonly  adopted  is  to  cast  a  small  flat  rectangular 
ingot  about  •£  inch  to  |  inch  thick,  which  is  quenched  in  water 
and  then  broken  in  half ;  if  the  steel  is  sufficiently  hot,  the 
fracture  will  exhibit  a  needle-shaped  structure  radiating  from 


METHODS   OF   MELTING  AND   EEFINING   COLD   CHARGES      141 

the  bottom  and  sides,  and  showing  the  usual  plane  of  junction 
running  at  an  angle  of  45°  from  the  bottom  corners  of  the  frac- 
tured surface.  Both  these  methods  have  the  disadvantage  that 
their  accuracy  is  entirely  dependent  upon  the  manner  in  which 
the  spoon  sample  is  taken,  and  are  therefore  too  subject  to  pos- 
sible error.  The  best  method,  which,  however,  is  less  economi- 
cal, consists  of  plunging  a  clean  |-inch  diameter  iron  rod  into 
the  bath  to  touch  the  bottom,  and  holding  it  immersed  for  a 
period  of  five  to  seven  seconds  according  to  the  character  of  the 
steel,  at  the  same  time  gently  moving  it  in  an  axial  direction. 
The  rod,  when  rapidly  withdrawn,  will  indicate  the  temperature 
of  the  bath  from  top  to  bottom  by  the  extent  to  which  the  steel 
has  either  adhered  to  it  or  cut  into  it,  as  the  case  may  be. 
Generally  speaking,  for  low  medium  to  high  carbon  steels  the 
bar  should  be  just  left  clean  after  immersion  for  five  seconds. 
The  first  methods  of  taking  temperature  may  well  serve  as  a 
guide  during  the  refining  operation,  but  the  actual  casting  tem- 
perature is  more  accurately  judged  by  the  rod  test. 

Calculation  of  Additions. — As  a  starting  point  for  calculations 
of  carbon  and  metallic  additions,  the  approximate  weight  of 
the  bath  of  steel  must  be  known.  There  is  always  a  certain 
melting  loss  which  will  vary,  according  to  the  nature  of  the 
scrap  used,  from  3  per  cent,  up  to  13  per  cent,  or  even  more,  the 
average  for  heavy  turnings  being  roughly  7  per  cent.  This 
melting  loss  can  only  be  determined  by  observation  of  losses  in 
previous  heats  when  similar  scrap  was  melted.  The  gross 
weight  of  steel  cast,  less  the  weight  of  all  ferro-alloy  additions 
made,  when  deducted  from  the  weight  of  scrap  charged,  will 
give  the  loss  incurred  during  the  entire  operation  with  sufficient 
accuracy.  The  following  examples  show  the  method  of  calculat- 
ing the  additions  for  a  high  carbon  and  a  chrome  steel  heat,  the 
same  system  being  adopted  for  any  variety  of  alloy  steel. 

Example  I. — It  is  required  to  make  a  high  carbon  steel  to 
the  following  specification  : — 

C         .         .         .  I'O   per  cent. 

Mn     .         .         .         .       '25 

Si        .         .         .  :     r      '15       „ 

P  below       .         ,         .   -   '02 

S  -02 


142  THE    ELECTEO-METALLUBGY   OF    STEEL 

Weight  of  scrap  charged        .....    6000  Ib.  •] 

Assume  a  7  per  cent,  melting  loss. 
Deduct  weight  lost  in  melting  (calculated)    .         .      420   ,, 

Actual  weight  of  steel  in  bath  at  skimming  .    5580    ,, 

Estimated  carbon  content  of  bath     .     '06  per  cent. 
Eequired  to  carburise  up  to       ,         .     '95    ,,       ,, 
(i.e.  *05  per  cent,  less  than  specifi- 

cation). 
/.  increased  per  cent.  C  to  add  to  bath  "89  per  cent. 

•89  x  5580  ., 
i.e.  weight  of  carbon  to  be  added  to  bath  = 


If  anthracite  containing  90  per  cent,  fixed  carbon  is  to  be 
used  for  carburising  the  bath,  and  the  carbon  absorption  is 
assumed  to  be  50  per  cent., 

then  weight  of  anthracite  to  be  used 

_  '89  x  5580  x  IQQ  x  100 

100  x  50  x  90 
=  110  Ib. 

Supposing  the  bath  sample  taken  for  carbon  and  manganese 
analysis  after  proper  formation  of  the  finishing  slag  is  found  to 
contain, 

C        .         .     '92  per  cent. 

Mn    .         .     '08 

then  the  carbon  will  have  to  be  raised  a  further  *08  per  cent. 
and  the  manganese  '17  per  cent,  before  tapping. 

For  the  addition  of  manganese  there  is  the  choice  of  either 
spiegel  or  ferro-manganese,  in  which  the  relative  percentages  of 
manganese  and  carbon  contained  are  as  4  to  1  and  11  to  1 
respectively. 

Since  only  '17  per  cent.  Mn  is  required  to  be  added,  it  is 
obvious  that  spiegel  is  preferable  to  use,  as  roughly  '04  per  cent. 
carbon  can  at  the  same  time  be  added. 

Increased  per  cent.  Mn  to  add  to  bath   =  .     17  per  cent. 

Per  cent.  Mn  in  spiegel       .         .         .   =  21*0          ,, 

Per  cent.  C  in  spiegel  .         .         .   =     5*0          ,, 

mu  ,  17  x  5580  x  100 

Then  weight  of  spiegel  required 

=  45  Ib. 


METHODS  OF  MELTING  AND  REFINING  COLD  CHARGES   143 

The  bath  will  now  contain  '92  per  cent.  +  -04  per  cent.  C 
or  '96  per  cent.  C,  and  is  therefore  still  too  low  for  the  specifica- 
tion. 

The  most  accurate  means  of  increasing  the  carbon  content 
is  by  the  addition  of  white  iron  or  haematite  pig-iron,  which  can 
be  obtained  with  not  more  than  1*5  per  cent.  Si,  although  2'5 
per  cent,  is  a  more  general  figure.  Assume,  however,  that 
white  iron  is  to  be  used  containing  4*5  per  cent.  C,  then  since 
the  finished  steel  is  to  contain  1  per  cent.  G  the  white  iron  only 
contains  3*5  per  cent.  C  available  for  addition  to  the  bath. 

Now  the  calculated  carbon  content 

of  the  bath  after  the  addition  of 

spiegel =  -96  per  cent.  C. 

/.  per  cent.  C  to  be  added  by  the 

white  iron  .         .         .         .         .  =  04  per  cent.  C. 

,     ,..     .  -04  x  5580  x  100  „ 

.  \  the  wt.  of  white  iron  required     .  =  -  -  Ib. 

3'5  x  100 

=  64jb. 

The  weights,  therefore,  of  carburisers  and  ferro-manganese 
required  are  :— 

Anthracite  (added  after  skimming)          .  110  Ib. 
Spiegel  (added  5  minutes  before  tapping)    45   ,, 
White  iron  (added  before  the  spiegel)      .    64  ,, 

In  the  above  calculations  it  will  be  seen  that  no  allowance 
has  been  made  for  the  increase  in  the  weight  of  the  charge  due 
to  the  anthracite  and  ferro-manganese  additions  ;  the  increase 
is  so  small  that  the  error  is  negligible.  In  the  case  of  the  white 
iron  addition,  the  carbon  content  is  only  just  over  four  times  the 
specification  figure,  so  that  the  calculations  have  to  be  based  on 
the  carbon  available  for  carburising.  The  same  remarks  might 
equally  well  apply  to  the  carbon  added  by  spiegel,  but  the 
carbon  content  is,  in  this  case,  slightly  higher  and  the  actual 
weight  of  alloy  less,  so  that  the  discrepancy  is  negligible  when 
compared  to  the  crude  estimation  of  melting  loss  which  always 
varies  from  heat  to  heat.  It  has  been  previously  stated  that  no 
loss  of  alloys,  added  after  the  formation  of  a  white  finishing 
slag,  is  allowed  for  in  the  calculations. 


144  THE    ELECTKO-METALLUKGY   OF    STEEL 

Example  II.  —  It  is  required  to  make  a  chrome  steel  to  the 
following  specification : — 

C          .         .1*2  per  cent. 
Cr        .         .3-0 
Mn       .         .       -4 

Assume  the  same  initial  charge  and  melting  loss  as  in  the 
previous  example : — 

The  actual  weight  of  steel  in  the  bath  at  skimming  =  5580  Ib. 
For  the  addition  of  chromium  suppose  there  are  two  grades  of 
ferro-chrome  available,  both  containing  65  per  cent.  Cr,  but  one 
with  5  per  cent.  C  and  the  other  9 '2  per  cent.  C.  These  are 
the  two  commonest  brands  of  ferro-chrome  made,  and  it  will  be 
seen  to  what  use  the  varying  carbon  contents  can  be  put  for 
the  final  adjustment  of  carbon,  while  adding  the  correct  amount 
of  chromium.  Suppose  equal  chromium  equivalents  of  the  two 
alloys  are  used,  and  the  per  cent,  carbon  added  by  them  is 
calculated,  then,  if  the  bath  sample  after  carburising  has  a 
carbon  content  which,  together  with  the  carbon  added  by  one 
grade  of  the  ferro-chrome,  would  be  either  above  or  below  the 
specification  figure,  the  correct  adjustment  may  possibly  be 
made  by  varying  the  proportions  of  the  high  and  low  carbon 
grades  used.  In  this  example,  however,  an  addition  of  manga- 
nese is  necessary,  so  that  there  is  a  still  further  means  of  adjust- 
ing the  carbon  by  the  addition  of  either  spiegel,  ferro-manganese, 
or  a  mixture  of  the  two. 

It  will  be  supposed  that  it  is  preferable  to  use  only  the  4  per 
cent,  to  6  per  cent.  C  ferro-chrome,  which  for  this  specification 
will  have  05  per  cent,  minus  3  per  cent.  Cr  and  5  per  cent, 
minus  1*2  per  cent,  carbon  available  for  addition  to  the  bath. 

Before  calculating  the  quantity  of  anthracite  for  carburising, 
it  is  necessary  to  know  about  how  much  carbon  will  be  added 
by  the  ferro-chrome  and  spiegel.  Spiegel  has,  in  this  case, 
been  chosen  in  preference  to  ferro-manganese  for  the  purpose  of 
calculation,  so  that  in  the  event  of  the  carbon  found  in  the  bath 
sample  being  lower  or  higher  than  was  intended,  there  is  still 
a  possibility  of  adjustment  by  using  the  high  carbon  ferro- 
chrome  in  place  of  the  4  per  cent,  to  6  per  cent.  C  grade,  or 
ferro-manganese  in  place  of  spiegel  respectively. 


METHODS    OF   MELTING   AND   REFINING    COLD   CHARGES      145 

If  both  the  lower  carbon  alloys  were  used,  and  the  carbon 
was  found  to  be  too  high  in  the  bath  after  carburising,  then 
there  would  be  no  means  of  reducing  it  to  within  the  specifica- 
tion limits.  Therefore,  since  carbon  can  easily  be  added  by  white 
iron  or  haematite  pig  additions,  it  is  always  a  safer  policy  to 
purposely  under-carburise  slightly  with  anthracite,  and  rely  upon 
alloys  and,  if  then  necessary,  white  iron  or  pig  for  the  subse- 
quent and  final  carburisation.  Calculating  the  ferro-chrome 
first  :— 

Wt.  of  steel  in  bath   .         .         .   =  5580  Ib. 
Per  cent,  of  Cr  required  in  bath   =•  3  per  cent. 

Then  weight  of  Cr  to  add  (using  the  4  per 

3  x  5580  x  100 
cent.  -  6  per  cent.  C  grade)  .   =   1QQ  x  (65  _  ^ 

=  270  Ib. 

Since  the  available  percentages  of  Cr  and  C  in  the  ferro- 
chrome  are  as  62  to  3*8,  it  follows  that  an  addition  of  3  per 

62 
cent.  Cr  is  accompanied  by  an   addition   of  3  -r  ^    or    '184 

per  cent.  C. 

Since  spiegel  is  being  used  for  the  addition  of  say  "3  per 
cent.  Mn  (assuming  '1  per  cent  in  the  bath), 

Then  the  carbon  added  will  equal  "07  per  cent,  (about). 

Assume  per  cent.  C  in  the  bath  before  skimming  =  *06  per  cent, 
and  per  cent.  C  added  in  ferro-chrome  .         .  =  18        „ 
and  per  cent  C  added  in  spiegel   .         .         .=  '07        „ 
Then  the  total  carbon  in  the  bath,  if  the  above  additions  are 

made  without  any  addition  of  anthracite,  would  be  "31  per  cent. 

Now  per  cent.  C  required  by  specification    .  =  1'2    percent. 
Total  per  cent.  C  in  bath  with  alloy  additions  =    '31       ,, 

.".  per  cent.  C  to  add  as  anthracite        .         .  =    '89       ,, 
Actually  it  is  safer  to  aim  rather  lower  than  the  calculated 
figure.     Then  per  cent.  C  to  be  added        .         .  =  '8  (say) 

Now  weight  of  steel  in  bath  at  skimming  =  5580  Ib. 
Per  cent.  C  required  to  add  as  anthracite  =  '8  per  cent. 
Then  wt.  of  anthracite  required,  assuming  90  per  cent.  C  in 
the  anthracite  and  50  per  cent.  C  absorption 

10 


146    .  THE   ELECTRO-METALLURGY   OF    STEEL 

•8  x  5580  x  100  x  100 

100  x  50  x  90 
=  99  Ib. 

Supposing  the  bath  sample,  taken  after  formation  of  the 
finishing  slag,  was  found  to  contain — 

C         .         .     '79  per  cent. 
Mn     .         .12 

The  per  cent.  C  in  the  bath  is  lower  than  was  expected  from 
calculation,  and  it  will  be  necessary  to  use  the  high  carbon 
ferro-chrome  in  place  of  the  lower  carbon  grade  either  wholly 
or  in  part.  The  spiegel  addition  must  therefore  be  calculated 
first. 

Per  cent.  Mn  found  in  bath  .         .  =  '12  per  cent. 

,,         Mn  required  by  specification  =  '4          ,, 
„         Mn  to  be  added  as  spiegel      .  =  '28        ,, 
It   is   known  that  about    120   Ib.    of   ferro-chrome  will  be 
added  later,  so  that  this  weight  may  be  added  to  the  weight  of 
the  bath. 

Original  wt.  of  bath  at  skimming          .  =  5580  Ib. 
wt.  of  ferro-chrome  added       .  =     120    ,, 
wt.  of  bath  for  Mn  calculation  =  5700    ,, 
The  above  allowance  is  really  unnecessary  in  practice,  and 
is  done  in  this  example  to  convey  the  principle  which  must  be 
applied  in  the  case  of  large  additions  of  alloys. 

^T.      .      .  '28  x  5700  x  100  ., 

Wt.  of  spiegel  required  =  -  -  Ib. 

lUU   X    AL 

=  76  Ib. 

Since  the  Mn  content  of  spiegel  is  about 
four  times  the  carbon,  the  per  cent.  C 
added  by  spiegel  .  .  .  .  .  =  -28  -f-  4 

=     '07  per  cent. 

Now  per  cent.  C  found  in  the  bath  sample   =     '79  per  cent, 
and  per  cent.  C  added  as  spiegel      .         .  =     '07       ,, 

.'.  Total  carbon  without  addition  from  ferro- 
chrome  .         .         .         .         .         .         .  =     '86       ,, 

Per  cent.  C  required  by  specification  .         .  =  1*2         ,, 


.'.  C  to  be  added  by  the  ferro-chrome  .         .  =     "34  per  cent. 


METHODS   OF   MELTING   AND   REFINING   COLD   CHARGES      147 

From  this  figure  it  is  obvious  that  the  bulk  of  the  ferro- 
chrome  must  be  added  as  the  high  carbon  alloy  in  which  the 
available  Cr  and  C  are  (65  -  3)  per  cent,  and  (9'2  -  1/2)  per 
cent,  respectively,  the  available  carbon  being  therefore  practic- 
ally one-eighth  the  chromium.  If  this  alloy  only  is  used,  and 
per  cent.  Cr  added  as  ferro-chrome 

=  3  per  cent.  (i.e.  specification  figure), 
then  per  cent.  C  added 

by  this  alloy  =3^-8 

=  '37  per  cent. 

But  only  '34  per  cent.  C  is  actually  required,  so  that  a 
portion  of  the  high  carbon  grade  must  be  replaced  by  the  lower 
grade.  Since  the  per  cent.  Cr  is  the  same  for  both,  the  weight 
of  the  alloys,  if  mixed,  will  still  be  the  amount  previously  found, 
i.e.  127  Ib.  Trying  a  proportion  of  100  Ib.  high  carbon  alloy, 
and  27  Ib.  of  the  lower  carbon  grade,  and  knowing  that  127  Ib. 
adds  3  per  cent.  Cr  to  the  bath, 
then  per  cent.  Cr  added  by  the  higher  C  grade 

100  x  3 
=  — ™ —  =  2'36  per  cent.  Cr, 

and  per  cent.  C  added  by  the  higher  C  grade 

=    '29  (i.e.  one-eighth  the  chromium). 
Again  the  per  cent.  Cr  added  by  the  lower  C  grade 

X     O  nr*r 

'63o  per  cent.  Cr, 

and  per  cent  C  added  by  the  lower  C  grade  =  '04  per  cent. 
.  *.  Total  carbon  added  by  the  mixed  alloys  =  "33  per  cent.  C. 
The  above  proportion  of  the  two  alloys  is  near  enough. 
The  additions,  then,  for  the  charge  will  be  : — 

Wt.  of  anthracite  for  carburising        .  =     99  Ib. 
Wt.  of  8-10  per  cent.  C  ferro-chrome  =  100    „ 
Wt.  of  4-6  per  cent.  C  ferro-chrome   .  =    27   ,, 
Wt.  of  Spiegel    .         .         .         .•        .  =     76   „ 
Tapping. — There  are  a  few  points,  which  might  be  mentioned 
in  connection  with  pouring,  that  may  materially  assist  in  pre- 
venting any  deterioration  in  the  quality  and  composition  of  the 
steel  during  transfer  from  the  furnace  to  the   ladle.     A  basic 
reducing  slag,  if  in  proper  condition,  is  creamy  in  consistency, 


148 


THE   ELECTKO-METALLUEGY   OF   STEEL 


and  has  no  power  of  cohesion,  as  in  the  case  of  a  vitreous  or 
siliceous  slag.  The  particles  are  easily  broken  up,  and,  owing 
to  their  high  degree  of  inf usibility,  do  not  readily  escape  from  a 
mass  of  molten  steel,  when  once  entrapped.  It  is,  therefore, 
preferable  to  hold  back  the  slag  when  pouring  into  a  ladle,  and 
only  allow  it  to  pass  over  the  spout  with  the  last  portion  of 
steel.  To  effect  this  without  running  any  risk  of  the  slag  ad- 
hering to  the  banks  and  remaining  in  the  furnace,  a  small 
addition  of  fluor  spar  may  be  given  just  before  pouring,  in  order 
to  mechanically  increase  its  fluidity  at  a  reduced  temperature. 
In  many  cases  a  special  brick  or  a  tapping  spout  (Fig.  80)  is 

used,  which  holds  back  the 
slag  until  the  steel  has  passed 
over  into  the  ladle.  It  is  also 
a  good  practice  to  hold  the 
ladle  for  five  minutes  before 
teeming,  as  this  will  offer  an 
opportunity  for  entrapped 
slag  and  gases  to  rise.  The 
spout  should  always  be  per- 
fectly dry,  so  that  there  may 
be  no  risk  of  the  steel  boil- 
ing on  it,  and  becoming  oxi- 
dised. 

Furnace  Tools  and  Manipulation. — The  tools  shown  in 
Fig.  81  are  used  to  conduct  the  operations  of  fettling,  skimming, 
slag  sampling  and  charging  heavy  pieces  of  scrap.  The  dimen- 
sions shown  are  suitable  for  a  7-ton  furnace,  and  will  be  rather 
less  for  smaller  furnaces.  The  skimming  rakes  should  be  used 
with  care,  as  under  the  best  conditions  they  require  frequent 
renewal  of  blades.  If  the  bath  is  cold,  the  rakes  will  become 
covered  with  steel,  and,  if  too  hot,  will  be  badly  cut  away  unless 
withdrawn  in  time.  The  hooked  bar  is  useful  for  clearing  scrap 
off  the  banks,  and  for  taking  slag  samples.  The  fettling  shovel 
is  best  made  as  shown,  so  that-  it  can  be  drawn  back  on  to  the 
furnace  door  sills  without  lifting. 

Fettling. — Fettling  is  a  most  important  operation,  upon 
which  not  only  the  life  of  a  lining  depends,  but  also  the  ease 
with  which  the  proper  metallurgical  conditions  can  be  main- 


Fia.  80. 


METHODS   OF   MELTING  AND  REFINING   COLD   CHARGES      149 

tained.  Fresh  dolomite  crushed  to  pass  a  f  inch  or  £  inch 
mesh,  and  free  from  slaked  dust,  should  be  used.  Care  must 
be  taken  that  the  dolomite  thrown  on  to  the  slag  line  does  not 
roll  down  the  banks  and  build  up  the  bottom,  which  is  a  common 
error  with  unskilled  furnacemen.  The  slag  line  is  always 
fettled  as  far  as  possible,  and  then  the  furnace  is  charged  with 
scrap  up  to  that  line.  Turnings  or  other  small  scrap,  when 
used,  are  best  charged  round  the  foot  of  the  fettled  slag  line, 
and  will  form  a  seating  on  which  more  dolomite  can  be  banked 


\Z/  S<~-~G     RAK.        iB'-o'tONO 


SAKWUMO    SOOOM 


D 


FIG.  81. 

up.  Dolomite  can  then  be  used  liberally,  and,  with  judicious 
fettling,  may  be  built  up  as  a  facing  to  the  lower  part  of  a 
badly-cut  silica  wall  and  so  greatly  prolong  its  life.  The  door 
jambs  and  spout  also  require  very  careful  attention,  the 
former  being  kept  in  shape  with  crucible  ganister  or  magnesite 
powder  mixed  with  just  enough  clay  to  make  it  bind  ;  the 
spout  is  best  lined  with  any  kind  of  moulding  sand,  which  is 
thrown  on  after  removal  of  the  slag,  and  beaten  down  to  the 
correct  shape.  Ganister,  although  largely  used,  is  not  so  good, 
as  it  requires  far  more  drying  and  cracks  when  dry,  while 
moulding  sand,  with  a  small  percentage  of  water,  only  requires 


150  THE   ELECTEO-METALLUEGY   OF   STEEL 

a  good  skin  drying  and  will  not  cause  the  steel  to  boil  up  on 
passing  over. 

ACID  PEOCESS. 

The  chemistry  of  steel-making  by  the  acid  process  is  very 
similar,  whether  it  is  conducted  in  electric  or  gas  furnaces,  or 
even  in  the  converter.  In  each  case  the  removal  of  carbon, 
silicon,  and  manganese  from  the  raw  material,  whether  it  be 
pig-iron,  steel  scrap,  or  a  mixture  of  the  two,  follows  from 
simple  oxidation,  which,  except  in  the  converter,  proceeds 
under  the  direct  influence  of  iron  oxides  and — to  a  more 
limited  degree — oxygen  in  the  furnace  atmosphere.  The  acid 
process,  as  in  the  case  of  the  basic  process,  may  be  employed 
for  the  conversion  of  steel  scrap  into  ingots  or,  more  generally, 
castings,  as  well  as  for  refining  molten  oxidised  steel. 

The  process,  as  applied  to  the  working  of  cold  charges,  may 
be  briefly  divided  into  three  stages  :— 

(i)  Melting  down  under  oxidising  conditions. 

(ii)  Boiling  out  carbon  under  an  oxidising  slag. 

(iii)  Addition  of  alloys  and  finishings. 

General  Outline. — In  the  acid  process  there  is  no  reducing 
period  during  which  dissolved  oxides  and  sulphur  are  removed, 
and  it  is  therefore  far  more  important  than  in  basic  working  to 
melt  under  conditions  that  do  not  conduce  to  over-oxidation 
of  the  bath.  One  great  advantage  is  the  economy  resulting 
from  the  use  of  only  one  slag,  which  makes  it  possible  to  pro- 
duce a  rather  larger  output  than  is  possible  with  basic  furnaces 
of  similar  power  capacity.  If,  on  the  other  hand,  it  is  necessary 
for  purposes  of  carburising  to  remove  the  first  slag  and  then 
form  another,  this  economy  will  naturally  disappear. 

The  acid  process  is  more  especially  suitable  for  the  manu- 
facture of  castings  from  raw  materials  (usually  steel  scrap) 
sufficiently  free  from  both  phosphorus  and  sulphur  to  meet  the 
required  specification.  Steel  made  for  foundry  purposes  will 
not  contain  as  a  rule  more  than  '35  per  cent.  C,  so  that  its  manu- 
facture is  possible  without  any  carburising  addition  other  than 
ferro-alloys ;  this  makes  it  possible  to  operate  with  one  slag,  so 
that  the  full  benefit  of  the  process  is  derived  in  this  particular 


METHODS   OF   MELTING   AND   KEFINING    COLD   CHARGES       151 

application.  To  prevent  over-oxidation  of  the  bath  during 
melting,  it  is  advisable  that  the  charge  should  contain  a  sufficient 
quantity  of  carbon,  so  that  when  entirely  melted  there  may  still 
be  at  least  '3  per  cent.  C  in  the  bath.  Mild  steel  scrap,  which 
would  melt  to  form  an  over-oxidised  bath  with  a  carbon  content 
below  this  figure,  is  usually  mixed  with  a  small  quantity  of  pig- 
iron  or  preferably  carbon  dust,  which,  being  absorbed  on  melt- 
ing, helps  to  limit  the  extent  of  bath  oxidation.  Manganese 
and  silicon  are  removed  by  the  oxide  of  iron,  which  is  either 
added  to  the  slag  in  the  form  of  ore,  or  results  from  oxidation  of 
the  scrap  in  the  furnace.  Carbon  will  also  be  reduced  until  the 
slag  becomes  so  impoverished  in  metallic  oxides  that  further 
reaction  becomes  impossible.  The  carbon  remaining  in  the 
bath  after  fusion  of  the  charge  may  be  further  reduced  by 
additions  of  ore  to  the  slag,  until  it  is  sufficiently  low  for  the 
final  addition  of  spiegel  or  ferro-manganese  to  bring  the  bath 
within  the  specification  figures  for  carbon  and  manganese. 
Before  describing  in  detail  the  different  operations  during  the 
melting  down  and  finishing  of  a  charge  of  scrap,  it  may  be  ad- 
visable to  indicate  briefly  the  consecutive  steps  in  the  furnace 
manipulation,  and  the  order  and  nature  of  the  chemical  reactions 
that  take  place. 

The  following  description  will  apply  more  particularly  to 
furnaces  having  a  capacity  of  not  more  than  three  tons,  but, 
with  slight  modification  of  the  details  of  manipulation,  will  be 
equally  applicable  to  larger  furnaces  :— 

1.  Hand    charging    of    scrap    into    the    previously    heated 
furnace. 

2.  Doors  closed,  load  on  and  melting  begins. 

3.  Scrap  melts  under  the  electrodes,  forming  pools  of  metal 
covered  by  a  slag  formed  from  the  siliceous  fettling  material, 
dirt,  etc.     Oxidation  of  carbon,  manganese  and  silicon  begins,  if 
sufficient  oxide  present. 

4.  Small  quantity  of  ore  and  other  fluxes  added  to  the  slag, 
if  necessary;  chemical  reactions  proceed  slowly. 

5.  Melting  proceeds  until  bulk  of  scrap  has  melted  to  form 
a  large  bath. 

6.  Further  addition  of  scrap  made  and  melting  proceeds. 


152  THE   ELECTRO-METALLURGY  OF   STEEL 

7.  Kepetition  of  (6),  if  necessary. 

8.  Entire   charge   melted;    carbon    in    bath    too   high    for 
finishing,  and  Mn  and  Si  low.     Addition  of  ore  made. 

9.  Boil  begins  and  carbon  is  removed,  until  the  slag  becomes 
impoverished  in  iron  oxide  and  the  boil  subsides. 

10.  Further  ore  added,  if  carbon  is  still  too  high. 

11.  Carbon  content  of  bath  sufficiently  low ;  iron  oxide  in 
slag  reduced  to  a  figure  incapable  of  producing  further  oxidation 
of  carbon. 

12.  Temperature   of    the   bath   adjusted    as   required,    and 
finishing  additions  made  before  casting. 

13.  Load  off;  steel  poured. 

Choice  of  Scrap. — Scrap  suitable  for  the  acid  process  must 
not  contain,  when  melted,  more  phosphorus  and  sulphur  than 
the  specification  of  the  steel  demands.  Carbon  contamination 
is  not  such  a  serious  matter  in  the  case  of  acid  scrap  for  the 
reasons  already  indicated,  but  should  not  exceed  a  degree  that 
would  unduly  lengthen  the  boiling  down  operation  after  com- 
plete fusion  of  the  charge.  Foreign  matter  and  dirt  present  in 
carelessly  collected  and  stored  scrap  will  usually  be  siliceous  in 
character,  and  is  therefore  less  harmful  to  an  acid  lining.  On 
the  other  hand,  the  scrap  should  preferably  be  clean  and  free 
from  rust,  so  that  it  may  be  melted  to  a  bath  containing  not 
less  than  *3  per  cent,  to  '4  per  cent.  C  and  a  minimum 
amount  of  dissolved  oxide  of  iron. 

With  regard  to  the  shape,  size,  and  other  physical  conditions 
which  influence  the  ease  of  the  furnace  manipulation  and 
mechanical  control,  the  remarks  which  have  already  been  made 
in  reference  to  scrap  suitable  for  the  basic  process  will  equally 
apply. 

Method  of  Charging. — The  method  of  charging  miscellaneous 
scrap  in  a  manner  most  favourable  to  the  maintenance  of  a 
steady  load  and  other  desirable  conditions  is  substantially  the 
same  for  both  acid  and  basic  processes,  but  it  should  be  noted 
that  pig-iron  or  carbon  dust,  when  used,  is  generally  mixed 
with  the  scrap  near  the  bottom  in  the  initial  charge.  This  is  done 
so  that  the  carburising  action  may  proceed  as  soon  as  the 
metal  forms  a  pool,  and  thereby  prevent  an  undue  absorption 
of  oxide  as  melting  continues, 


METHODS   OF   MELTING  AND  EEFINING   COLD   CHARGES      153 

Formation  and  Function  of  Acid  Slags. — The  functions  of  an 
acid  slag  are  twofold  : — 

(i)  To  reduce  the  carbon,  manganese,  and  silicon  in  the  bath 
to  a  desired  percentage. 

(ii)  To  serve  as  an  inert  covering  of  the  bath  of  steel 
while  ferro-alloy  additions  are  being  made,  or  whenever  it 
is  desired  to  prevent  further  chemical  reaction  taking  place 
before  pouring. 

The  essential  constituents  of  an  active  acid  slag  are  SiO2, 
FeO  and,  generally,  small  quantities  of  CaO,  A12O3,  MnO,  and 
Fe203.  The  Si02  is  derived  from  the  acid  hearth  and  the 
loose  sand  or  ganister  used  as  a  fettling  material.  A  small 
quantity  of  lime  is  sometimes  added  after  the  preliminary 
formation  of  the  slag,  generally  for  the  purpose  of  thinning 
it  when  the  FeO  content  has  fallen  to  a  low  figure. 

There  is  usually  sufficient  iron  oxide,  derived  from  the  rust 
on  the  scrap  or  from  oxidation  while  melting,  to  combine  with 
the  silica  for  the  formation  of  a  slag  covering.  Other  bases, 
such  as  A12O3  and  CaO,  will  be  readily  absorbed  by  such  a  ferrous 
•  silicate  slag  which,  owing  to  its  high  silica  content,  is  powerfully 
acid  in  character. 

If  the  scrap  used  is  exceptionally  clean  and  the  oxidation 
loss  during  melting  very  small,  it  may  be  necessary  to  add  iron 
ore  after  a  small  bath  of  steel  has  been  formed.  This  is  done 
to  provide  the  iron  oxides  necessary  for  the  removal  of  carbon, 
silicon,  and  manganese,  and  at  the  same  time  to  open  out  or 
thin  the  highly  siliceous,  pasty  slag;  the  free  oxygen  of  the 
Fe203  plays  an  important  part  in  the  oxidation.  The  removal 
of  carbon  from  the  bath,  which  is  always  evidenced  by  a  distinct 
boil  due  to  evolution  of  carbon  monoxide  gas,  only  proceeds 
where  the  temperature  of  the  slag  and  steel  is  above  the  reaction 
temperature.  Accordingly,  the  oxidation  of  carbon  only  occurs 
in  the  neighbourhood  of  the  arcs,  until  the  entire  bath  with  its 
slag  covering  is  hot  enough  for  the  reaction  to  take  place  over 
the  whole  surface.  An  absence  of  "  boil"  during  the  melting 
down  period  does  not,  then,  necessarily  indicate  lack  of  iron  oxides 
in  the  slag.  This  latter  condition  can  best  be  judged  by  the 
colour  of  a  broken  sample.  It  has  been  previously  stated  that 
the  carbon  content  of  the  charge  and  the  amount  of  FeO  and 


154  THE    ELECTKO-METALLUKGY.    OF    STEEL 

Fe2O3  passing  into  the  slag  or  added  to  it  as  ore,  is  usually  so 
balanced  that  the  carbon  content  remaining  in  the  bath  after 
complete  fusion  of  the  charge  will  be  about  '3  per  cent.,  this 
being  done  to  prevent  over-oxidation.  As  the  removal  of  carbon 
from  the  bath  proceeds,  the  slag  becomes  impoverished  in 
metallic  oxides,  until  finally  it  may  be  no  longer  capable  of  pro- 
moting oxidation,  and  the  boil  ceases.  This  reduction  of  the 
oxides  is  accompanied  by  very  marked  changes  in  the  appear- 
ance of  the  slag  when  cold,  and  enables  any  subsequent  addition 
of  ore,  for  the  further  removal  of  carbon  to  a  desired  figure, 
to  be  correctly  estimated. 

An  acid  slag  capable  of  boiling  out  carbon  will  usually  con- 
tain over  25  per  cent,  of  combined  FeO  and  MnO,  which  is  rather 
less  than  in  open  hearth  practice.  The  percentage  of  silica  in 
the  final  slag  will  also  be  much  higher,  so  that  rather  more  lime 
is  sometimes  needed  to  obtain  the  required  fluidity.  The  pasti- 
ness of  the  slag  at  tapping  is  a  quality  that  is  very  useful  in 
foundry  practice  where  ladle  lip-pouring  is  adopted,  as  the  slag 
can  be  so  readily  held  back  in  the  ladle  or  skimmed  off. 

The  varying  percentage  of  carbon  in  a  bath  during  the  "  boil " 
can  be  judged  by  observing  the  fracture  of  bath  samples  ;  such 
tests  are  generally  made  until  the  carbon  is  thought  to  be  nearly 
low  enough,  when  a  test  sample  is  sent  to  the  laboratory  for  a 
carbon  determination.  Under  proper  conditions  of  working,  the 
slag  should  cease  to  be  active  and  become  almost  incapable  of 
further  carbon  oxidation,  just  when  the  percentage  of  carbon  in 
the  bath  has  fallen  to  the  required  figure.  This,  of  course,  re- 
quires considerable  skill  and  judgment  when  making  the  addi- 
tions of  iron  ore  during  the  boiling  down  period.  If  the  carbon 
is  reduced  below  the  desired  figure  it  can  at  once  be  raised  by 
the  addition  of  pig-iron,  a  sufficient  quantity  being  added  to 
increase  the  carbon  in  the  bath  by  the  desired  amount,  and  at 
the  same  time  to  compensate  for  any  subsequent  loss  by  slag 
reaction  ;  by  this  means  the  correct  bath  and  slag  conditions 
may  be  simultaneously  obtained. 

The  elimination  of  carbon  by  boiling  gradually  proceeds  in 
successive  stages,  following  each  of  the  several  small  additions 
of  iron  ore  which  are  made  from  time  to  time  to  promote  the 


METHODS   OF   MELTING   AND   BEFINING   COLD   CHARGES      155 

reaction.  These  additions  are  made  more  cautiously  as  the 
carbon  approaches  the  desired  percentage. 

Summarising  the  foregoing  description  of  slag  formation  and 
reaction,  it  will  be  seen  that  the  necessity  of  carefully  control- 
ling the  metallic  oxides  present  in  the  slag  is  even  greater  than 
in  the  basic  process,  where  over-oxidation  of  the  bath  can  be 
almost  completely  corrected  under  a  reducing  slag  without  the 
aid  of  the  final  ferro-alloy  additions. 

Physical  and  Chemical  Characteristics  of  Acid  Slags. — The 
colour  and  fluidity  of  electric  furnace  acid  slags  change  very 
markedly  during  the  period  of  boiling.  Assuming  there  is  *3  per 
cent,  to  *4  per  cent,  carbon  in  the  bath  of  steel  after  complete 
fusion  of  the  charge,  there  should  then  be  almost  sufficient  avail- 
able metallic  oxides  in  the  slag  to  reduce  the  carbon  to  the 
desired  percentage.  The  oxides  present  in  such  a  quantity  will 
colour  the  slag  light  or  dark  brown,  but,  as  the  slag  becomes 
impoverished  in  these  oxides,  the  colour  of  the  slag  fracture  will 
progressively  alter.  In  the  regular  operation  of  a  furnace 
carrying  the  same  quantities  of  steel  and  slag  in  successive  heats, 
the  colour  of  the  slag  may  be  used  to  indicate  the  percentage  of 
carbon  which  it  is  still  capable  of  boiling  out  without  further 
ore  addition.  This  being  the  case,  it  is  important  to  examine 
the  slag  at  frequent  intervals  and,  by  knowing  the  approximate 
carbon  content  at  a  particular  moment  by  a  fracture  test,  it  is 
possible  to  estimate  fairly  well  just  how  much  ore  to  add  for  the 
further  removal  of  carbon  required.  In  colour  the  slag  succes- 
sively passes  through  various  shades  of  yellow,  until  it  finally 
becomes  very  faintly  blue-green,  and  is  then  no  longer  capable  of 
producing  a  vigorous  boil.  The  viscosity  will  become  greater 
by  the  partial  removal  of  the  iron  oxide  base,  and  may  then  be 
adjusted  by  small  additions  of  some  other  base,  such  as  lime. 
The  considerable  viscosity  of  electric  furnace  acid  slags  is  mainly 
due  to  the  high  percentage  of  silica,  which  frequently  reaches 
65  per  cent. 

A  typical  analysis  of  an  electric  furnace  acid  slag  just  before 
tapping  is  as  follows  : — 


156  THE   ELECTROMETALLURGY    OF    STEEL 

Si02  ....     64'0    per  cent. 

FeO  ....     12-36 

ALA  ....       7-27        „ 

MnO  ....     10-62 

CaO  .         .      '   .         .5-9 

MgO  ....     Trace 

10015  per  cent. 

The  combined  FeO  and  MnO  are  considerably  lower  than 
in  acid  open  hearth  slags,  which  indicates  that  the  oxidation  of 
carbon  may  be  effected  with  a  less  oxidised  slag  and,  probably, 
with  less  tendency  towards  over-oxidation  of  the  steel. 

Ferro= Alloy  Additions. — Alloy  additions  are  calculated  and 
made  in  the  same  way  as  in  the  basic  process,  with  the  excep- 
tion that  10  per  cent,  to  20  per  cent,  loss  of  Mn  and  Si  must  be 
allowed  for,  without,  however,  counting  on  any  loss  of  the  carbon 
added.  The  addition  of  spiegel,  ferro-manganese,  and  ferro- 
silicon  is  made  only  after  the  slag  is  pale  green,  and  low  in 
active  oxides.  The  bath  will  then  exhibit  only  a  very  slight 
boil,  and  the  carbon  content,  either  determined  or  calculated,  is 
no  longer  liable  to  appreciable  variation.  The  carbon  added  to 
the  bath  as  a  constituent  of  the  ferro-alloys  will  not  suffer  any 
loss,  but,  owing  to  the  slight  oxidation  of  the  bath  at  the  end 
of  the  boil,  there  will  be  a  small  loss  of  manganese  and  silicon, 
which  is  allowed  for  as  above.  The  proper  allowance  to  be 
made  for  loss  on  this  account  and  by  a  slight  slag  reaction  can 
easily  be  determined  by  experience. 


CHAPTEK  VIII. 

LIQUID  STEEL  REFINING. 

IN  the  preceding  chapters,  the  methods  by  which  high  grade 
steel  is  made  from  miscellaneous  steel  scrap  have  been  dealt 
with  and  classified  according  to  the  acid  or  basic  character  of 
the  slag  used.  The  chemical  reactions  upon  which  the  basic 
and  acid  processes  depend  are  mainly  due  to  the  interaction  of 
molten  slag  and  metal,  and  therefore  proceed  independently  of 
the  action  of  fusion.  The  conditions  under  which  melting  takes 
place  certainly  influence  the  chemical  character  both  of  the  bath 
of  metal  and  of  the  slag,  and  in  this  way  alone  does  the  process 
of  melting  affect  the  chemical  reactions  which  follow,  when  the 
temperature  and  chemical  conditions  of  the  bath  and  slag  are 
satisfied. 

For  these  reasons,  the  process  of  making  highly  refined  steel 
from  cold  charges  may  be  regarded  simply  as  a  process  of  melt- 
ing, followed  by  a  period  during  which  conditions  suitable  for 
chemical  reactions  are  maintained,  although  in  practice  the  two 
phases  proceed  simultaneously.  In  liquid  refining  the  melting 
phase  is  eliminated  and  the  chemical  phase,  when  conducted 
under  oxidising  or  non-oxidising  slags,  may  be  considered  as 
identical  in  its  functions,  irrespective  of  how  the  molten  steel 
has  been  produced.  The  advantage  of  liquid  refining  is  economic 
rather  than  technical,  as  cheaper  methods  of  producing  liquid 
steel  can  be  employed  than  is  possible  by  melting  scrap  with 
electric  energy.  The  electric  furnace,  if  basic  lined,  may  be 
used  for  purposes  of  dephosphorising  followed  by  carburising, 
desulphurising  and  deoxidising  treatments,  and  when  acid  lined, 
is  used  for  the  sole  object  of  deoxidising  liquid  steel  already  low 
enough  in  phosphorus  and  sulphur. 

Liquid  Refining  in  Basic  Furnaces. — The  refining  operations 
usually  include  removal  of  phosphorus  as  a  preliminary  to 

(157) 


158  THE    ELECTEO-METALLURGY   OF    STEEL 

carburising  and  the  subsequent  treatment  for  the  removal  of 
sulphur  and  dissolved  oxides.  Sometimes  the  liquid  steel  may 
not  require  further  elimination  of  phosphorus,  in  which  case  it 
is  carburised  in  the  transfer  ladle,  or  in  the  electric  furnace 
itself,  prior  to  the  addition  of  fluxes  for  the  formation  of  a 
reducing  slag.  When  cold  blown  acid  bessemer  steel  is  elec- 
trically refined  for  further  removal  of  phosphorus,  it  is  some- 
times necessary  to  add  about  '3  per  cent,  carbon  to  the  steel  in 
the  transfer  ladle,  so  as  to  minimise  the  risk  of  skulling  by 
causing  a  slight  lowering  of  the  melting-point ;  this  practice  is 
generally  followed  when  the  time  taken  in  transfer  is  consider- 
able. 

Both  bottom  teeming  and  lip  pouring  ladles  are  used  for 
transferring  the  liquid  steel,  the  latter  type  being  used  only 
when  the  slag  covering  is  viscous  and  easily  skimmed.  In  either 
case,  a  special  launder  is  used  to  convey  the  steel  from  the  ladle 
to  the  electric  furnace,  so  that  the  steel  may  flow  clear  of  the 
door  sills  on  to  the  hearth.  When  a  dephosphorising  treatment 
is  necessary,  both  iron  oxide,  in  the  form  of  iron  ore  or  mill 
scale,  and  lime  are  shovelled  into  the  furnace  whilst  the  steel  is 
still  being  poured.  These  fluxes  quickly  fuse  and  form  a  suit- 
able basic  oxidising  slag  under  which  phosphorus  removal  is 
rapidly  promoted.  When  the  phosphorus  removal  has  been 
carried  far  enough  the  bath  is  skimmed  and  carburised,  if  neces- 
sary ;  fresh  fluxes  are  then  added,  and  the  process  of  deoxidation 
and  desulphurising  followed  in  the  same  manner  as  described 
in  Chapter  VII.  The  table  on  the  opposite  page  gives  particulars 
of  the  materials  used,  time  occupied,  and  power  consumption  per 
ton  of  metals  charged  for  three  typical  heats,  in  each  of  which 
about  11  tons  of  liquid  bessemer  steel  were  dephosphorised  and 
finished  in  a  basic  lined  electric  furnace. 

The  power  was  supplied  up  to  the  time  of  skimming  at 
about  2000  K.V.A.,  and  reduced  to  a  much  lower  figure  during 
the  last  part  of  each  heat. 

The  following  typical  analysis  of  the  steel,  before  and  after 
refining,  shows  the  extent  to  which  the  quality  is  improved. 


LIQUID   STEEL   EEFINING 


159 


Analysis  of  Bessemer  Steel. 

C  '05  --10    percent. 

Mn  -05  —10 

Si  -005— -015 

S  -035— -07 

P  -095 


Analysis  of  Finished  Steel. 


'09  per  cent. 

•025  -  O35  per  cent, 

•015  -  -04 


I. 

II. 

III. 

Weight  bessemer  steel  in  Ib.  . 

26,860 

25,060 

25,900 

»        Scale 

700 

800 

600 

i»        Ore                     „  „ 

100 

900 

400 

,,        Lime                  ,,  ,, 

800 

800 

600 

Current  on     . 

8  hrs.  15  mins. 

4  hrs.  30  mins. 

3  hrs.  10  mins. 

Began  skimming   . 

9  hrs.  5  mins. 

5  hrs.  30  mins. 

3  hrs.  50  mins. 

Finished  skimming 

9  hrs.  15  mins. 

5  hrs.  40  mins. 

4  hrs.  10  mins. 

Time  for  skimming 

10  mins. 

10  mins. 

20  mins. 

Weight  crushed  electrode  (for 

carburising)  . 

150 

— 

— 

,,          Lime 

700 

800 

600 

„          Fluor  spar 

325 

290 

275 

Coke  dust  (added  to 

slag)    .         .         . 

200 

270 

150 

,,          Ferro  -manganese    . 

270 

200 

150 

„          Ferro-silicon,  50  per 

cent. 

86 

90 

20 

Nickel 

— 

820 

— 

„          Other  ferro-alloys    . 

— 

— 

580 

Time  tapped 

10  hrs.  10  mins. 

7  hrs.  0  mins. 

5  hrs.  0  mins. 

Total  time     .           -      . 

1  hr.  55  mins. 

2  hrs.  30  mins. 

1  hr.  50  mins. 

Units  consumed 

3100 

4200 

2600 

Unit  per  ton  charged     . 

255 

360 

218 

The  removal  of  sulphur  and  phosphorus  can  always  be 
carried  to  the  lower  limit  when  specified,  and  the  silicon  can 
also  be  kept  low  if  necessary.  The  physical  properties  of 
electrically  refined  mild  steel  show  an  increase  of  15  per  cent, 
in  the  ultimate  strength  as  compared  with  basic  open  hearth 
steel,  but  at  the  same  time  the  elongation  is  decreased  by  11  per 
cent.  ;  this  comparison  is  shown  in  a  table  of  tests  compiled  by 
C.  G.  Osborne  for  a  paper  read  before  the  American  Electro- 
chemical Society  in  1911. 

Liquid  Refining  in  Acid  Furnaces. — The  theory  and  practice 
of  deoxidising  liquid  steel  in  an  acid-lined  electric  furnace  has 
been  carefully  studied  both  in  America  and  Germany.  Various 
theories  have  been  advanced  to  explain  the  exact  manner  by 
which  deoxidation  proceeds,  but  the  several  suggested  chemical 


160  THE   ELECTRO-METALLURGY   OF   STEEL 

reactions  responsible  for  the  removal  of  oxygen  by  this  process 
are  not  substantiated  by  any  conclusive  proof.  Thallner,  in 
1913,  advanced  an  ingenious  theory  to  explain  the  physical 
characteristics  of  cast  steel,  based  upon  physical  suppositions 
rather  than  upon  chemical  composition.  According  to  his 
theory  the  quality  of  steel  was  greatly  influenced  by  the  size  of 
the  molecules  when  in  the  liquid  condition,  the  physical  pro- 
perties being  improved  the  smaller  their  size.  This  particular 
condition  of  a  bath  of  steel  was,  he  considered,  not  only  a  func- 
tion of  temperature,  but  also  dependent  upon  the  combined  effect 
of  several  chemical  reactions,  in  which  carbon  plays  an  essential 
part.  The  reactions  which  promote  the  deoxidation  of  steel  in 
an  acid-lined  furnace,  according  to  this  theory,  are  briefly  as 
follows  : — 

(a)  Keduction  of  silicon  from  the  acid  hearth,  with  the 
formation  of  silicon  carbide. 

(6)  Decomposition  of  the  silicon  carbide  by  metallic  oxides 
dissolved  in  the  steel  and  present  in  the  slag,  with  formation 
of  iron  carbide. 

(c)  Partial  decomposition  of  the  iron  carbide  by  the  oxides 
of  the  slag. 

The  silicon  reduced  from  the  lining  does  not  then  remain  in 
the  steel,  but  indirectly  causes  the  formation  of  iron  carbide  to 
which  is  ascribed  the  specially  fine  grain  obtained.  This  is 
only  possible  so  long  as  oxide  of  iron  is  present  in  the  slag  or 
bath  in  sufficient  quantity  to  split  up  the  silicon-carbide  first 
formed,  otherwise  silicon  will  be  reduced  from  the  lining  in 
increasing  quantity  and  remain  in  the  finished  steel.  Accord- 
ing to  the  process  evolved  on  the  above  assumption  and  practised 
at  the  works  of  the  Lindenburg  Steel  Company  at  Eemscheid- 
Hasten,  Germany,  the  carbon  necessary  for  the  above  reaction 
is  introduced  into  the  bath  in  the  form  of  briquettes,  consisting 
of  carbon  and  iron  filings  or  borings.  The  reduction  of  silica  to 
silicon  is  mostly  from  the  hearth  lining  and  only  to  a  lesser 
degree  from  the  slag,  the  reduction  in  this  latter  case  being  in- 
fluenced by  the  temperature,  the  amount  of  carbon  present  in 
the  bath,  and  the  percentage  of  silica  in  the  slag. 

From  the  results  obtained  by  melting  and  refining  cold 
charges  in  an  acid-lined  three  ton  Girod  furnace  at  Gutehoffnung- 


LIQUID   STEEL   REFINING  161 

shiitte,  the  following  conclusions  have  been  drawn  regarding 
the  conditions  which  influence  the  reduction  of  silicon  during 
the  deoxidising  period  : — 

(a)  Provided  that  carbon  is  present  in  the  bath  the  silicon 
reduction  will  be  almost  entirely  derived  from  the  acid  hearth, 
and  only  to  a  lesser  degree  from  the  slag. 

(b)  The  reduction  of  silica  to  silicon  from  the  slag  may  be 
increased  by  raising  its  silica  content. 

(c)  The  reduction  of  silica  to  silicon  from  the  slag  may  be 
considerably  influenced  by  any  excessive  rise  of  temperature. 

(d)  The   reduction   of   silica   to  silicon   is  considerably  in- 
fluenced by  the  amount  of  carbon  in  the  bath,  subject  also  to 
temperature. 

The  process  practised  at  these  works  consisted  of  cold  melt- 
ing, followed  by  a  careful  refining.  The  first  or  oxidising  slag  is 
similar  to  acid  open  hearth  slag  containing  a  high  percentage 
of  mixed  iron  and  manganese  oxides.  The  charge  after  melting 
down  is  skimmed  and  fresh  slag  added  consisting  of  about  75 
per  cent,  crushed  silica  brick  and  25  per  cent,  lime,  to  which  is 
later  added  sufficient  manganese  oxide  to  give  about  10-15  per 
cent,  in  the  final  slag  ;  this  practice  of  using  a  finishing  slag  low 
in  metallic  oxide  is  comparable  to  true  liquid  refining  of  steel 
transferred  from  some  other  steel  furnace,  and  is  quite  distinct 
from  the  cruder  method  of  melting  and  finishing  cold  scrap 
charges  under  one  slag  as  described  in  Chapter  VII.  The  bath 
of  steel  should  contain  rather  less  than  the  final  required  per- 
centage of  carbon  before  addition  of  the  deoxidising  slag. 

The  following  analyses  are  given  as  typical  of  the  deoxidis- 
ing or  finishing  slags  used  at  Gutehoffnungshiitte  : — 

Si02  54-4  52-6  54-2  66-27 

CaO  11-5  13-7  23-2  15-26 

MgO  5-2  3-2  3-05  2-03 

ALjOj  1-72  1-52  1-86  -15 

FeO  3-35  3-85  4-05  2-81 

MnO  23-55  23-7  11-15  11-68 

S  -29  -13  -4  -51 

P,O5  03  -12 

The  advantages  of  acid  refining  lie  in  the  rapidity  of  de- 
oxidation,  reduction  of  lining  repair  costs  due  to  the  cheaper 
price  of  acid  materials  used,  and  the  prolonged  life  of  the  silica 
roof.  The  power  consumption  is  also  less  than  for  basic  refining, 

11 


162  THE   ELECTRO-METALLURGY   OF   STEEL 

being  about  100  kw.  -hours   per  ton  for  a  15-ton  furnace,  the 
period  of  refining  being  about  1-j-  hours. 

It  has  been  generally  admitted  that  deoxidation,  promoted 
under  an  acid  slag  by  actual  silicon  reduction,  is  far  more  com- 
plete and  produces  better  results  than  when  done  in  the  more 
rapid  and  cruder  manner  by  additions  of  ferro-silicon.  Deoxida- 
tion in  the  acid  furnace  is  comparable  to  the  "  killing"  action 
or  "  dead  melting  "  associated  with  crucible  steel  manufacture. 
Thalmer's  theories,  which  are  based  upon  the  initial  production 
of  silicon  carbide,  might  reasonably  be  applied  to  the  crucible, 
but  he  points  out  that  the  "  killing  reaction  cannot  be  so  com- 
plete in  the  latter  case  owing  to  the  much  lower  silica  content 
of  the  clay  material  ". 

The  silicon-carbide  theory  is  not  generally  accepted,  and  it  is 
difficult  to  reconcile  it  with  the  fact  that  the  minimum  tempera- 
ture of  silicon  carbide  formation  is  not  less  than  1800°  C.,  a 
temperature  that  can  only  be  reached  in  the  arc  zones. 

The  theory  generally  favoured  depends  rather  upon  the 
alternate  formation  and  dissociation  of  ferro-silicon  due  to  the 
combined  interaction  between  silica,  carbon,  and  iron  oxide. 
It  is  probable  also  that  metallic  iron  itself,  when  in  contact  with 
a  highly  siliceous  slag  and  in  the  presence  of  carbon,  may  pro- 
mote the  reduction  and  absorption  of  silicon  at  temperatures 
below  the  reduction  temperature  of  silicon  from  pure  silica.  In 
either  case  the  silicon  would  not  be  reduced  in  its  elemental 
form,  but  as  a  compound  of  iron  and  silicon.  The  silicon  thus 
entering  a  bath  of  steel  will  immediately  reduce  any  metallic 
oxides  in  solution,  and  in  this  manner  cleanse  the  bath  of  these 
impurities. 

Acid  refining  based  upon  the  above  theory  has  been 
largely  practised  for  the  deoxidation  of  basic  open  hearth  steel. 
Liquid  steel,  carburised  to  within  a  few  points  of  the  specifica- 
tion figure,  is  transferred  to  the  electric  furnace,  and  fluxes  con- 
sisting of  iron  ore,  lime,  and  sand  are  charged ;  the  greater 
proportion  of  the  silica,  however,  is  derived  from  the  fettling 
material,  which  becomes  detached  from  the  banks  and  hearth. 
The  slag  becomes  bluish  when  the  steel  is  hot,  and  the  silicon  in 
the  bath  should  not  be  above  *05  to  '08  per  cent.  After  the 
bath  becomes  deoxidised  the  silicon  content  will  rapidly  rise, 


LIQUID    STEEL    REFINING  163 

especially  if  the  steel  should  be  very  hot,  and  for  this  reason 
the  charge  should  be  tapped  without  delay.  This  method  of 
refining  enables  any  class  of  carbon  or  alloy  steel  to  be  made, 
which  is  perfectly  deoxidised  and  free  from  segregation  in  the 
ingot 

Apart  from  the  chemical  effect  of  liquid  refining,  the 
improvement  due  to  purely  physical  reasons  must  not  be  dis- 
regarded. It  has  been  stated  elsewhere  that  considerable  im- 
portance is  now  attached  to  allowing  finished  liquid  steel  to 
remain  in  a  perfectly  quiescent  condition  for  some  time  previous 
to  casting,  this  being  done  for  the  sole  purpose  of  allowing 
finely  suspended  slag,  or  other  foreign  matter  of  low  specific 
gravity  and  gases,  to  rise  and  pass  out  of  the  steel.  The  de- 
oxidation  of  steel  in  electric  furnaces  is  not  accompanied  by 
any  commotion  due  to  evolution  of  carbon  monoxide,  so  that 
during  the  process  of  refining  the  bath  is  also  in  a  condition 
physically  suitable  for  the  free  separation  of  minute  slag  particles. 

Scope  and  Application  of  Liquid  Refining. — Liquid  refining 
in  electric  furnaces  frequently  constitutes  the  final  stage  of 
what  are  commonly  known  as  Duplex  and  Triplex  processes. 
These  processes,  as  their  names  imply,  embrace  two  or  three 
distinct  operations  for  the  manufacture  of  finished  steel,  each 
operation  being  conducted  in  separate  furnaces.  When  basic 
lined  electric  furnaces  are  used,  the  liquid  steel  may  be  simply 
crude  blown  metal  produced  by  the  acid  or  basic  bessemer  pro- 
cess ;  in  either  case  the  blown  steel  may  possibly  require  further 
dephosphorising,  which  can  be  done  either  in  the  basic  electric 
furnace  prior  to  deoxidation  and  desulphurisation,  or  in  some 
other  furnace  before  transfer  to  the  electric.  In  the  former 
case  the  process  is  Duplex,  and  in  the  latter  case  Triplex. 

The  Duplex  or  Triplex  process  is  used  for  the  production  of 
high  class  steel  from  unrefined  liquid  steel,  such  as  is  made  by 
the  basic  bessemer  process  in  Europe  or  the  acid  bessemer  in 
America.  In  both  cases  the  liquid  product  may  be  electrically 
refined  to  produce  a  steel  equal  in  quality  to  that  made  by  the 
more  general  method  of  melting  and  refining  scrap.  The  basic 
open  hearth  furnace  has  also  been  used  for  producing  cheap 
liquid  dephosphorised  steel,  requiring  subsequent  deoxidation 
and  desulphurising  only  in  the  basic  electric  furnace. 


164  THE    ELECTRO-MET ALLUKGY   OF    STEEL 

The  application  of  electric  furnaces  to  liquid  refining  is 
essentially  suited  to  large  outputs  and  rapid  operation.  The 
electric  furnace  should  always  be  operated  at  a  high  load  factor, 
and  to  render  this  possible  a  regular  and  frequent  supply  of 
liquid  steel  must  be  provided  for.  The  bessemer  process 
fulfils  these  conditions,  and  it  is  in  conjunction  with  this  method 
of  steel-making  that  the  electric  furnace,  as  used  for  liquid 
refining,  has  been  most  generally  applied.  Tilting  basic  open- 
hearth  furnaces  working  a  continuous  process,  such  as  the 
Talbot,  are  equally,  if  not  more  suitable  than  bessemer  con- 
verters, since  the  phosphorus  can  be  sufficiently  reduced  to 
meet  any  acid  open-hearth  carbon  or  alloy  steel  specification, 
which  thus  shortens  the  period  of  subsequent  refining  in  the 
electric  furnace. 

From  technical  standpoints  a  Duplex  or  Triplex  process, 
which  embodies  a  final  refining  treatment  of  semi-finished  steel 
by  the  electric  process,  is  perfectly  feasible,  as  has  been  con- 
clusively demonstrated  both  in  America  and  Germany.  The 
possibilities  of  liquid  refining  must  be  studied  rather  from  an 
economic  standpoint,  and  in  this  direction  they  will  be  depen- 
dent upon  the  following  factors  : — 

I.  The  production  of  cheap  liquid  steel  which  can  be  en- 
hanced in  value  by  further  refining. 

II.  A  frequent   and   regular  supply   of   liquid  steel  to  the 
electric   furnace,    so    that  the  load  factor  and  output  may  be 
raised  to  a  maximum,  and  all  overhead  charges  correspondingly 
reduced. 

III.  A   suitable   market   for   the   electrically   refined   steel, 
which   in  the  case  of  large  outputs  must  be  able  to  compete 
favourably  with  the  higher  grades  of  bessemer  and  open-hearth 
steels. 


CHAPTEE  IX. 

INGOT  CASTING. 

Theory  of  Ingot  Formation. — The  art  of  steel-making,  as 
applied  to  the  manufacture  of  ingots,  has  for  its  ultimate  object 
the  production  of  steel  in  a  crude  form  that  will  submit  to 
subsequent  physical  or  mechanical  treatments,  such  as  forging, 
rolling,  machining,  and  heat  treatment,  without  exhibiting  or 
developing  any  structural  defect.  Liquid  steel  of  excellent 
quality  may  be  rendered  quite  unsuitable  for  the  purpose  in- 
tended by  improper  methods  of  casting  and  handling,  and  it  is, 
therefore,  of  the  utmost  importance  to  adopt  a  method  of 
casting  that  is  satisfactory  for  each  particular  class  of  steel  and 
shape  of  ingot.  For  instance,  the  existence  of  a  long  pipe  may 
be  harmless  for  one  variety  of  steel,  but  might  entirely  ruin  an  in- 
got of  another  variety  having  imperfect  welding  properties.  The 
solidification  of  steel  in  a  cast-iron  mould  has  been  the  subject 
of  considerable  investigation  and  discussion  of  recent  years. 
More  and  more  attention  is  being  paid  every  day  to  this  par- 
ticular branch  of  the  art  of  steel-making,  and  even  now  the 
theories  advanced  by  those  who  have  long  and  carefully  studied 
the  matter  are  by  no  means  unanimous  in  all  respects. 

Iron  is  an  element  that  possesses  a  definite  crystalline  form, 
so  that  solidification  of  steel  first  proceeds  by  crystallisation  of 
the  elementary  iron  from  the  molten  metal,  which  contains 
other  elements  in  solution.  It  is  first  necessary  to  study  the 
process  of  solidification  of  a  body  of  liquid  steel  under  different 
thermal  conditions,  as  this  has  a  very  important  bearing  upon 
the  crystalline  structure  and  chemical  constitution  of  the 
solidified  mass.  Solidification  naturally  takes  place  wherever 
the  temperature  has  fallen  to  the  freezing-point  of  the  steel,  and 
may  consequently  be  marked  by  an  isothermal  zone,  which  must 
progressively  travel  in  an  inward  direction  from  all  boundary 

(165) 


166  THE   ELECTROMETALLUKGY   OF   STEEL 

surfaces  exposed  to  the  influence  of  cooling.  The  rate  at  which 
such  a  zone  travels  at  any  moment  is,  of  course,  a  measure  of 
the  speed  of  actual  solidification,  and  depends  upon  the  tem- 
perature of  the  liquid  steel  and  the  rate  at  which  heat  is  being 
abstracted. 

Should  the  liquid  steel  be  at  a  temperature  well  above  its 
freezing-point  and  subject  to  rapid  cooling,  then  the  isothermal 
zone  of  solidification  will,  at  any  moment,  be  sharply  de- 
fined and  localised  at  the  plane  of  junction  between  either  the 
liquid  steel  and  the  walls  of  the  mould,  or  the  liquid  steel  and 
an  already  solidified  envelope,  owing  to  the  abrupt  temperature 
difference  between  the  two  in  either  case.  Solidification  under 
these  thermal  conditions  will  proceed  by  the  constant  deposition 
of  thin  films,  or,  in  other  words,  by  the  slow  and  steady  inward 
growth  of  the  solidified  envelope  when  once  formed.  This 
mode  of  solidification  is  therefore  favoured  by  (a)  a  high  initial 
casting  temperature,  (b)  a  rapid  abstraction  of  heat. 

Now,  even  supposing  solidification  to  have  been  proceeding 
in  the  above  manner,  the  mean  temperature  of  the  still  liquid 
steel  will  have  been  steadily  falling  by  conduction  of  heat  out- 
wardly to  the  surrounding  walls,  and  might,  in  fact,  reach  a 
temperature  near  to  its  freezing-point  before  the  process  of 
solidification  had  proceeded  very  far.  In  that  event  there  will 
be  little  or  no  temperature  difference  between  the  surface  of 
the  truly  solid  and  liquid  portions,  and  the  isothermal  zone, 
instead  of  being  sharply  defined  as  before,  will  become  obliterated 
and  merge  into  the  liquid  steel.  Solidification  will  not  then 
proceed  in  a  distinct  and  well-defined  manner,  but  will  take 
place  more  or  less  irregularly  in  a  zone  of  much  greater  depth. 

Again,  if  steel  is  cast  into  a  mould  having  a  very  low 
conductivity  and  small  thermal  capacity,  it  will  not  be  subject 
to  sudden  chilling  at  its  boundary  surfaces,  and  will  fall  in  tem- 
perature slowly  as  a  whole,  until  such  a  point  is  reached  when 
a  rapid  irregular  solidification,  as  above  described,  will  result 
from  any  further  lowering  of  temperature ;  this  case  applies 
whether  the  steel  be  cast  hot  or  cold.  Solidification  may  also 
proceed  in  this  manner  when  steel,  at  a  temperature  only 
slightly  above  its  freezing-point,  is  cast  into  a  mould  of  good 
conductivity  and  high  thermal  capacity  ;  this  case  is  analogous 


INGOT   CASTING  167 

to  that  previously  described,  where  rapid  solidification  in  irregular 
zones  followed  well-defined  progressive  solidification.  Those 
thermal  conditions,  then,  which  favour  rapid  irregular  solidifica- 
tion are  (a)  a  low  initial  casting  temperature,  (b)  slow  abstrac- 
tion of  heat.  Either  of  the  above  conditions,  however,  if 
sufficiently  pronounced,  will  promote  this  mode  of  solidification, 
irrespective  of  the  other. 

A  careful  distinction  must  be  drawn  between  the  total  time 
of  solidification  and  the  actual  rate  at  any  moment,  otherwise 
those  thermal  conditions,  which  have  been  mentioned  as  con- 
ducive to  the  two  distinct  modes  of  solidification,  would  appear 
to  be  quite  erroneous  and  contrary  to  fact.  For  example,  sup- 
pose equal  weights  of  steel  at  the  same  temperature  to  be 
poured  into  an  iron  mould  of  large  heat  capacity  and  high 
conductivity,  and  a  sand  mould  of  similar  dimensions  having 
a  very  small  heat  capacity  and  low  thermal  conductivity. 
Obviously  the  actual  rate  of  solidification  in  the  iron  mould 
will  be  rapid  at  first  and  gradually  slow  down  until  the  mean 
temperature  of  the  still  liquid  steel  has  fallen  by  conduction 
almost  to  its  freezing-point,  after  which  it  becomes  more  rapid 
and  general,  as  previously  explained.  In  the  case  of  the  sand 
mould,  solidification  will  be  delayed  owing  to  the  low  thermal 
conductivity  of  the  material,  and  will  hardly  begin  to  proceed 
until  the  temperature  of  the  liquid  steel  as  a  whole  has  more 
nearly  approached  its  freezing-point ;  solidification  will  then 
proceed  rapidly  throughout  an  ill-defined  inwardly  progressing 
zone.  The  total  time  elapsed  between  the  moments  of  pouring 
and  complete  solidification  will,  of  course,  be  considerably  less 
in  the  case  of  the  iron  mould,  although  the  bulk  of  the  steel 
will  have  changed  from  the  liquid  to  solid  state  at  a  slower 
rate. 

It  has  been  already  stated  that  solidification  proceeds  by  the 
crystallisation  of  pure  iron  from  the  liquid  steel,  and  that  the 
process  of  crystallisation  is  itself  influenced  according  to  the 
manner  of  solidification  at  any  moment.  Having  dealt  with 
the  various  thermal  conditions  which  influence  the  actual  rate 
of  change  of  state  from  liquid  to  solid,  and  the  zone  in  which 
it  occurs,  it  is  now  possible  to  see  how  these  same  conditions 
affect  the  manner  in  which  this  change  occurs  or,  in  other 


168  THE   ELECTEO-METALLtJEGY  OF   STEEL 

words,  the  crystalline  structure  of  the  solid  steel.  Referring  to 
Fig.  82,  assume  a  mass  of  hot  liquid  steel  C  to  be  poured  on  to 
a  heavy  iron  chill  plate  D  of  indefinite  area,  and,  taking  a 
hypothetical  case,  assume  that  there  is  no  heat  loss  from  the 
upper  surface  of  the  liquid  steel.  The  thermal  conditions 
assumed  are  such  as  to  promote  solidification  in  well-defined 
zones.  The  rate  at  which  heat  is  abstracted  from  the  molten 
steel  is  variable;  in  the  first  instance,  there  is  a  rapid  with- 
drawal of  heat  by  the  chill  plate  D  dependent  upon  its  thickness 
and,  therefore,  heat  capacity,  the  exchange  of  heat  being  rapid 
by  virtue  of  the  high  thermal  conductivity  of  the  cast-iron, 
which  diffuses  the  heat  throughout  its  mass.  There  must  be, 
however,  a  gradual  falling  off  in  the  rate  of  heat  withdrawal  as 
the  chill  plate  becomes  hotter  and  finally  assumes  a  temperature 
of  equilibrium,  which  occurs  when  the  gain  of  heat  from  the 
steel  by  conduction  is  equal  to  the  loss  of  heat  by  radiation. 
A  thin  layer  of  steel  immediately  next  to  the  surface  of  the 
plate  is  subject  to  an  intense  chilling  effect,  and  solidifies  very 
rapidly  with  the  formation  of  minute  crystals,  giving  rise  to  a 
very  close-grained  crystalline  structure.  The  thin  layer  of 
solid  steel  thus  formed  slightly  lowers  the  rate  at  which  heat  is 
abstracted,  and  then  allows  solidification  to  proceed  in  well- 
defined  isothermal  zones,  which  favours  a  more  regular  and 
perfect  growth  of  crystals.  Since  the  heat  travels  from  the 
steel  to  the  chill  plate  in  a  definite  direction,  it  follows  that  the 
crystals  must  grow  regularly  in  the  opposite  direction,  always 
presenting  their  uppermost  end  to  the  still  liquid  steel;  this 
gives  rise  to  a  "  needle  "-like  or  columnar  structure,  as  shown  in 
Fig.  82.  Crystals  which  grow  in  this  manner  will  have  a  well- 
defined  relative  orientation,  and  are  known  as  "  chill  "  crystals. 
An  ingot  exhibiting  this  structure  is  often  said  to  be  "  scorched," 
as  it  only  results  when  the  steel  is  cast  very  hot.  While 
solidification  proceeds  in  this  way,  the  liquid  steel  is  losing 
heat,  and,  should  its  mean  temperature  approach  its  freezing- 
point,  solidification  will  then  become  irregular.  If  this  occurs, 
crystallisation  will  not  take  place  by  the  steady  growth  of  the 
solidified  surface,  but  may  proceed  by  the  formation  of  individual 
crystal  grains  within  an  ill-defined  zone  and  remote  from  this 
surface.  The  crystallisation  will  then  be  irregular  and  the 


INGOT    CASTING  169 

solidified  steel  will  be  built  up  of  crystal  grains  having  no  fixed 
orientation  relative  to  one  another.  The  columnar  structure 
will  give  place  to  a  granular  structure  consisting  of  so-called 
"  equiaxed  "  crystals ;  the  proportion  of  "chill  "  to  "  equiaxed " 
crystals  will  naturally  depend  upon  the  relation  between  the 
speed  of  solidification,  and  the  rate  at  which  the  still  liquid  steel 
loses  heat  by  conduction.  Slow  heat  abstraction  will  retard 
solidification  and  favour  a  gradual  fall  of  temperature,  and  the 
proportion  of  "  chill  "  to  "  equiaxed  "  crystals  will  become  less, 
as  indicated  by  the  two  sketches  shown. 

The  above  theory  may  now  be  applied  to  demonstrate  the 
crystalline  character  of  a  steel  ingot  cast  in  a  square  open-top 
ingot  mould,  standing  on  a  heavy  cast-iron  bottom  or  chill 
plate.  It  has  been  explained  how  crystallisation  proceeds 
uniformly  in  a  direction  at  right  angles  to  a  chilling  surface,  so 
that,  after  the  initial  freez- 
ing of  the  envelope,  the 
steel  will  solidify  in  suc- 
cessive layers  parallel  to 
one  another,  provided  the 

rate   of  heat   abstraction  is       Rapid  Heat  Abstraction."  Slow  Heat  Abstract™ 
everywhere    uniform.       If  FIG.  82. 

casting  and  cooling  condi- 
tions are  such  as  to  favour  the  formation  of  "  chill  "  crystals, 
it  is  evident  that  they  will  grow  inwards  from  the  four  vertical 
sides  and  bottom  of  the  mould;  the  crystals  growing  from 
any  two  adjacent  chilling  surfaces  will  be  at  right  angles,  and 
meet  obliquely  in  a  plane  which  lies  at  an  angle  of  45°  to  each 
surface,  if  the  rate  of  growth  should  be  uniform  from  both 
chill  faces.  Four  such  planes  will  also  be  formed  inclining 
upwards  from  the  bottom  edges  of  the  mould,  and  their  lines 
of  intersection  will  form  a  pyramid  or  truncated  pyramid, 
according  to  whether  the  "  chill "  crystals  penetrate  to  the 
centre  of  the  ingot  or  not.  In  the  case  of  small  ingots,  the 
growth  of  the  chill  crystals  may  be  so  rapid  that  they  will 
penetrate  to  the  heart  of  the  ingot  before  the  temperature  of 
the  steel  at  any  time  remaining  fluid  has  fallen  sufficiently  low 
to  admit  of  the  growth  of  equiaxed  crystals;  this  only  occurs 
when  the  steel  is  cast  very  hot.  The  photograph  of  a  broken 


170  THE   ELECTKO-METALLUEGY  OF   STEEL 

ingot  shown  in  Fig.  83  indicates  the  three  distinct  forms  of 
crystallisation  illustrated  in  Fig.  82. 

If  the  walls  of  a  mould  vary  in  thickness,  the  rate  of 
crystallisation  will  depend  upon  the  heat-absorbing  capacity  at 
any  section,  and,  by  increasing  the  thickness  of  metal,  the  con- 
ditions which  favour  the  growth  of  chill  crystals  are  intensified. 
Molten  steel,  adjacent  to  the  corner  of  an  ingot  mould  having 
sides  of  uniform  thickness  at  any  horizontal  section,  is  likewise 
subjected  to  more  intense  chilling  than  at  points  intermediate 
between  two  corners,  so  that  chill  crystals  are  more  developed 
near  the  corners  of  an  ingot.  It  has  been  well  established  that 
the  planes  marking  the  junction  of  chill  crystals  are  planes  of 
weakness,  a  fact  that  is  demonstrated  by  the  presence  of 
longitudinal  corner  cracks,  which  sometimes  occur  in  defective 
ingots. 

The  solidification  of  steel,  irrespective  of  the  manner  of 
crystallisation,  is  accompanied  by  a  shrinkage,  which  must  not 
be  confused  with  the  contraction  shrinkage  that  follows  later. 
The  volume  of  liquid  steel  filling  a  mould  is  greater  than  the 
volume  occupied  by  the  solid  steel,  so  that,  unless  solidification 
is  accompanied  by  a  depression  of  the  liquid  level  to  compensate 
for  the  difference  in  the  two  volumes,  cavities  will  of  necessity 
be  formed  in  the  ingot,  and,  moreover,  at  that  point  where 
solidification  is  finally  completed.  The  shape  of  the  zone 
marking  the  surface  of  demarcation  between  liquid  and  solid 
during  the  process  of  solidification,  together  with  the  correspond- 
ing shape  of  the  shrinkage  cavity,  may  cause  defects  in  ingots 
that  are,  however,  capable  of  being  mitigated,  if  not  almost 
prevented.  Contraction  cavities  and  gas  cavities  may  also  be 
formed  under  certain  conditions,  and  give  rise  to  defects  which 
may  only  become  apparent  during  the  later  stages  of  mechanical 
and  physical  treatment.  Segregation  of  impurities,  which  only 
occurs  to  a  minor  extent  in  highly  refined  electric  steel,  must 
not  be  disregarded.  The  relationship  between  solidification,  or 
more  properly  the  mode  of  crystallisation  promoted,  and  the 
chemical  constitution  of  an  ingot  is  dealt  with  later  under  the 
subject  of  segregation. 

Before  describing  actual  methods  of  ingot  casting,  which 
will  differ  according  to  whether  the  mould  is  filled  from  the 


FIG.  83. 


[To  face  p.  170. 


INGOT   CASTING  171 

top  or  bottom,  the  design  of  the  moulds  and  the  use  of  certain 
special  apparatus,  it  is  more  convenient  to  examine  the  character 
of  the  commoner  ingot  defects  and  the  particular  casting  con- 
ditions responsible  for  their  formation.  In  this  way  the  merits 
of  the  various  methods  of  casting  may  be  better  judged  in  so 
far  as  they  avoid  or  minimise  these  harmful  conditions. 

Ingot  Defects. — Piping. — In  the  case  of  steel  cast  into  chill 
moulds,  the  change  of  state  from  liquid  to  solid,  which  is  ac- 
companied by  shrinkage,  has  been  briefly  considered.  Without 
using  special  precautions,  the  uppermost  layer  of  molten  steel 
in  an  ingot  mould  will  become  solid  before  solidification  of  the 
interior  is  complete,  and  since  the  crust  so  formed  completes 
the  solid  envelope  surrounding  the  still  liquid  portion,  it  follows 
that  further  shrinkage  cannot  be  followed  by  a  corresponding 
self-adjustment  of  the  envelope,  with  the  result  that  a  cavity 
or  series  of  cavities  are  formed  in  the  interior.  The  shape  of 
the  main  cavity  depends  upon  the  manner  in  which  solidifica- 
tion has  proceeded,  and  this  again  is  influenced  by  the  taper  of 
the  mould  walls,  the  direction  of  taper  of  the  mould  itself,  and 
the  position  occupied  by  the  last  portion  of  steel  entering  the 
mould.  Such  a  cavity  is  generally  called  a  "  pipe,"  a  term 
which  is  more  literally  descriptive  of  its  character  when  it 
occurs  as  an  elongated,  inverted  cone,  with  its  base  close  to 
the  ingot  top.  To  study  the  formation  of  pipes  it  will  be  easiest 
to  consider  the  process  of  solidification  of  liquid  steel  when  cast 
into  an  open  top  mould  with  parallel  sides.  Fig.  84  represents 
the  solidification  of  an  ingot  in  successive  stages.  While  the 
mould  is  slowly  filling,  the  upper  walls  are  becoming  heated  by 
radiation  from  the  stream  of  steel  and  from  the  rising  column 
of  steel  in  the  mould  ;  when  the  mould  is  full,  the  bottom  of 
the  ingot  will  have  begun  to  solidify  before  the  top  has  even 
begun  to  chill,  so  that  the  isothermal  zone,  which  represents 
the  surface  of  solidification,  will,  a  few  moments  after  teeming, 
be  somewhat  as  shown  in  the  sketch  A.  It  is  apparent  that  the 
body  of  steel  immediately  below  the  top  crust  formed  will,  on 
shrinking  from  it,  be  protected  from  further  rapid  loss  of  heat 
by  radiation,  and,  since  it  was  the  last  portion  teemed,  will  tend 
to  be  the  last  portion  to  solidify  and  thus  serve  as  a  reservoir 
from  which  liquid  steel  is  constantly  drawn  off  as  shrinkage 


172 


THE    ELECTRO-METALLUKGY   OF   STEEL 


proceeds  until  finally  exhausted.  The  other  sketches  shown  in 
Fig.  84  illustrate  how  the  pipe  is  formed  by  the  thickening  of 
the  ingot  wall  from  below  the  crust  downwards,  accompanied 
at  the  same  time  by  constant  depression  of  the  still  liquid  steel. 
When  solidification  is  on  the  point  of  completion,  the  ingrowing 
walls  of  solid  steel  at  the  middle  and  lower  end  of  the  ingot  may 
be  almost  parallel,  and  if  these  walls  should  meet  at  certain 
points  between  which  liquid  steel  still  remains,  it  is  obvious 
that  shrinkage  of  those  isolated  portions  cannot  be  met  by  draw- 
ing off  from  the  larger  reservoir  above.  Therefore,  at  those  points 
there  will  also  be  found  long,  narrow  cavities  F  (see  sketch 


FIG.  84. — Solidification  in  parallel  moulds. 

D),  which  are  known  as  "secondary  pipes,"  and  are  en- 
tirely disconnected  from  the  primary  or  main  pipe  E  above. 
Frequently  the  main  pipe  is  bridged  across  by  one  or  more 
crusts  of  solid  steel,  which  have  formed  at  different  levels,  allow- 
ing the  still  molten  steel  below  to  recede  and  keep  pace  with 
shrinkage. 

The  shape  of  the  pipe  formed  may  clearly  be  modified  by 
either  retarding  or  hastening  the  freezing  of  the  steel  at  the 
upper  end  relatively  to  the  lower.  By  retarding  the  rate  of 
cooling  at  the  top  end,  the  zone  of  complete  solidification  from 
wall  to  wall  will  reach  a  higher  position  in  the  ingot  before  the 


INGOT    CASTING  173 

reservoir  of  liquid  steel  has  been  finally  exhausted ;  in  this  way 
the  pipe  may  be  considerably  shortened,  and  will  have  greater 
lateral  dimensions.  It  will  be  mentioned  later  how  this  may 
be  effected  in  practice.  By  more  rapid  solidification  of  the  upper 
ingot  walls,  the  reservoir  of  liquid  steel  will  rapidly  diminish  in 
volume  before  solidification  of  the  lower  portion  of  the  ingot  is 
complete  ;  under  such  conditions,  those  portions  of  the  ingot  still 
remaining  liquid  will,  on  solidification,  produce  elongated  shrink- 
age cavities,  or  "  secondary  pipes,"  in  a  far  more  marked 
degree. 

In  some  cases,  a  solid  crust  may  not  be  formed,  and  the  pipe 
caused  by  the  continual  shrinkage  of  the  steel  will  then  be  ex- 
posed to  the  air  and  become  coated  with  oxide.  Even  supposing 
such  an  oxide  coated  pipe  were  capable  of  welding,  the  steel  in 
the  immediate  vicinity  of  the  weld  would  be  partly  decarburised 
and  generally  inferior  to  the  rest  of  the  billet.  Pipes  formed 
below  solid  crusts  are  free  from  a  coating  of  oxide,  but  even 
then  piped  ingots  of  certain  steels  will  not  weld  up  perfectly 
during  the  forging  or  rolling  operations.  The  presence  and 
extent  of  piping  are  influenced  by  different  methods  of  casting, 
to  be  dealt  with  later. 

In  certain  heavy  engineering  work,  where  safety  and  relia- 
bility is  of  primary  importance,  the  pipe  is  either  removed  by 
trepanning  or  by  rejection  of  the  upper  portion  of  the  ingot  in 
which  it  is  situated.  Kejection  of  the  top  is  only  effective  in 
the  absence  of  secondary  pipes. 

Segregation. — Liquid  steel  may  be  regarded  as  a  complex 
mixture  of  iron,  carbon,  silicon,  manganese,  phosphorus,  sulphur, 
etc.,  in  which  the  metalloids  probably  exist  dissolved  in  the  iron 
in  a  colloidal  state.  It  is  now  generally  accepted  that,  at  the 
moment  of  incipient  solidification,  pure  iron  begins  to  crystallise 
out  from  the  mother  liquor  in  the  form  of  dendrites,  which  may 
be  regarded  as  minute,  acicular  or  needle-shaped  crystals. 
According  to  Stead  these  crystals  shoot  out  branches  at  right 
angles  corresponding  to  the  axes  of  a  cube,  and  the  branches 
themselves  undergo  growth  of  crystallisation  in  like  manner. 
The  mother  liquor,  from  which  these  so-called  dendrites  or 
crystallites  grow,  becomes  enriched  in  sulphur  and  phosphorus, 
with  a  corresponding  increase  of  fusibility.  The  bulk  of  this 


174  THE    ELECTBO-METALLUKGY   OF    STEEL 

more  fusible  liquid  becomes  entrapped  in  the  ever-multiplying 
crystal  branches,  which  eventually  become  closely  interlocked. 
If,  however,  sulphur  and  phosphorus  are  present  in  sufficiently 
large  quantities  in  the  mother  liquor,  they  will  form  fusible 
compounds  of  low  specific  gravity,  and  these  minute  particles, 
owing  to  their  fluidity,  coalesce  and  are  then  either  entrapped 
or  pushed  forward  to  that  zone  of  the  ingot  where  solidification 
last  takes  place.  By  virtue  of  their  low  specific  gravity,  the 
sulphur  and  phosphorus  compounds  will  at  the  same  time  tend 
to  rise  through  the  mother  liquor,  and  it  is  partly  for  this 
reason  that  drillings  taken  from  an  ingot  in  the  neighbour- 
hood of  a  primary  pipe  contain  more  sulphur  and  phosphorus 
than  elsewhere.  This  local  concentration  of  the  impurities  is 
called  "  segregation,"  and  is  favoured  by  high  casting  tempera- 
tures and  rapid  cooling.  These  latter  conditions,  it  has  been 
explained,  promote  the  growth  of  "  chill  crystals  "  which  entrap 
less  mother  liquor  as  crystallisation  proceeds.  The  mother 
liquor,  therefore,  becomes  more  and  more  enriched  in  impurities, 
and  should  any  fusible  compounds  separate  out  as  segregates, 
they  are  pushed  forward  by  the  slowly  advancing  chill  crystals. 
For  this  reason,  as  Brearley  has  pointed  out,  a  ring  of  segregates 
is  often  found  lying  at  the  boundary  of  the  chill  and  equiaxed 
crystal  zones.  Equiaxed  crystals,  on  the  other  hand,  which 
result  from  rapid  irregular  crystallisation,  entrap  the  mother 
liquor  in  situ,  and  so  prevent  pronounced  segregation.  Car- 
bon and  manganese  also  segregate,  but  in  both  cases  the  per- 
centage enrichment  in  the  zones  of  segregation  is  very  small 
compared  with  that  of  sulphur  and  phosphorus.  Sulphur 
segregates  in  the  form  of  a  fusible  manganese  sulphide,  so  that 
sulphur  and  manganese  enrichments  are  always  coincident. 

Segregation  is  usually  regarded  as  a  distinct  fault,  but  in 
cases  where  the  upper  third  of  a  large  ingot  is  rejected  on  account 
of  a  pipe,  or  where  the  centre  of  an  ingot  is  removed  by  tre- 
panning, it  then  becomes  a  virtue  to  be  encouraged. 

Ghost  Lines. — "  Ghost"  is  the  name  given  to  a  defect  which 
only  becomes  apparent  on  machining.  It  appears  as  a  whitish 
streak  in  the  steel,  always  following  the  direction  of  elongation 
under  forging  or  rolling  treatment.  The  term  "ghost"  is 
applied  to  such  flaws,  because,  owing  to  their  usually  extreme 


INGOT   CASTING  175 

thinness,  they  can  be  removed  with  ease  on  machining;  un- 
fortunately, the  presence  of  a  ghost  line,  which  may  itself  be 
easily  removed,  indicates  the  presence  of  others,  which  may  be 
situated  outside  the  range  of  mechanical  removal  and  are  a  source 
of  danger  when  the  steel  is  under  transverse  stress.  According 
to  Stead,  these  white  "  ghost  lines  "  are  carbonless  streaks  of 
ferrite  in  which  are  usually  embedded  lenticular  or  drawn  out 
inclusions  of  manganese  sulphide,  the  presence  of  which  also 
indicates  the  segregation  of  phosphorus  in  the  same  region. 
The  magnitude  of  the  ghost  lines  is  more  pronounced  in  the 
case  of  high  phosphorus  steels,  and  to  this  element  he  attaches 
the  power  of  expelling  carbon  from  the  zone  of  segregation  on 
slow  cooling.  Others  consider  that  the  primary  existence  of 
"ghost  lines  "  is  due  to  slag  inclusions,  which  act  as  nuclei  for 
the  secondary  crystallisation  of  ferrite,  or  for  the  deposition  of 
hard  cementite  (Fe3C),  according  to  whether  the  percentage  of 
carbon  is  below  or  above  '89  per  cent.,  which  is  the  percentage 
present  in  the  saturated  eutectoid  or  solid  eutectic  of  iron  and 
carbon.  These  slag  particles  are  drawn  out  on  forging,  and 
cause  repeated  deposition  of  soft  ferrite  or  hard  iron-carbide  on 
cooling  from  a  temperature  above  the  critical  range.  This 
theory  also  explains  the  persistence  of  "ghost  lines"  even  after 
repeated  annealing,  unless  the  heat  treatment  is  so  prolonged 
as  to  cause  an  actual  dispersal  of  the  drawn  out  string  of  nuclei. 
Fig.  101  shows  slag  inclusions  imbedded  in  soft  ferrite  areas  in 
a  sample  of  an  annealed  steel  casting. 

Lapping  and  Folding. — These  terms  apply  to  irregularities 
produced  on  the  skin  of  an  ingot  whilst  the  level  of  liquid  steel 
is  rising  in  the  mould.  When  an  ingot  is  being  top-poured, 
it  sometimes  happens,  especially  if  the  steel  is  teemed  cold,  that 
a  pasty  semi-solidified  crust  forms  on  the  top  of  the  molten 
steel  and  assumes  a  slightly  convex  form  along  the  edges  of 
contact  with  the  mould ;  this  convexity  increases,  and  the 
molten  steel  is  held  from  the  sides  of  the  mould  until  the  pres- 
sure becomes  great  enough  for  the  steel  to  burst  through  the 
pasty  crust.  The  molten  steel,  being  released,  immediately 
flows  round  a  portion  of  the  edge  of  the  crust  and  fills  the  gap 
between  its  convex  surface  and  the  side  of  the  mould.  It  is 
improbable  that  the  steel  flowing  over  the  convex  edges  of  the 


176  THE   ELECTEO-METALLUEGY   OF    STEEL 

oxidised  crust  ever  properly  welds  to  it ;  in  fact,  unless  the 
crust  has  been  momentarily  remelted  by  the  steel  flowing 
upon  it,  the  surfaces  of  contact  are  likely  to  become  sub- 
sequently detached  on  forging  or  rolling.  Even  in  the  event  of 
fusion,  the  crust  will  carry  into  the  steel  a  quantity  of  oxide, 
which  gives  rise  to  local  unsoundness.  In  top-poured  ingots 
the  lap  or  fold,  which  f  jllows  the  line  of  contact  between  the 
edge  of  the  crust  and  the  steel  flowing  over  it,  is.  generally  wavy 
and  irregular  in  form.  In  the  case  of  bottom-run  ingots,  the 
steel  rises  in  the  mould  with  less  surface  agitation  and  with 
uniform  pressure,  so  that  if  a  small  annular  crust  should  form, 
producing  only  a  slight  convexity  where  in  contact  with  the 
mould,  the  hot  steel  on  rising  will  flood  the  crust  uniformly,  so 
that  any  "  laps"  or  "  folds  "  exhibited  on  the  skin  will  be  close 
together  and  parallel.  If,  however,  the  steel  is  very  cold  and  the 
crust  that  forms  is  thick,  then  the  folds  will  be  wavy  as  in  the 
case  of  top-poured  ingots ;  the  horizontal  laps  cannot  be  pro- 
duced in  open  top  ingots  of  small  section,  owing  to  the  com- 
motion caused  by  the  stream  striking  the  steel  as  it  fills  the 
mould.  The  objection  to  fusion  of  the  crust  by  the  steel  flood- 
ing it  does  not  apply  so  strongly  to  closed  top  moulds,  where 
oxidation  of  the  semi-solid  crust  can  only  be  very  slight.  Some 
alloy  steels,  particularly  those  containing  chromium,  are  more 
liable  to  lap,  owing  to  the  readiness  with  which  oxidation  crusts 
form.  Folding,  then,  results  either  from  teeming  cold  steel  or 
from  teeming  too  slowly,  which  allows  the  steel  to  cool  off  and 
form  semi-solidified  crusts  while  filling  the  mould. 

"Shell"  or  "Catch". — If  steel  on  entering  an  open  top 
mould  should  either  strike  the  side  or  splash  up  against  it  from 
the  bottom  plate,  it  will  be  immediately  chilled  and  form  a  thin, 
irregular  shaped  strip.  This  will  either  adhere  to  the  side  of  the 
mould  or  be  enveloped  and  remelted  by  the  liquid  steel.  In  the 
former  case,  as  filling  proceeds,  the  liquid  steel  will  lise  and  chill 
against  this  strip  without  effecting  a  proper  weld.  The  irregular 
shaped  strip  remains  on  the  surface  of  the  ingot  as  a  "  catch,"  and 
will  sometimes  emit  a  hollow  sound  when  tapped,  indicating 
imperfect  adhesion  to  the  body  of  the  ingot.  In  the  latter  case 
the  ingot  is  said  to  be  "  shelled,"  and  will  usually  be  rejected 
as  scrap.  Even  if  the  "  catch  "  is  not  very  pronounced,  it  will 


INGOT    CASTING  177 

cause  local  unsoundness  and  small  cracks  owing  to  unequal  con- 
traction of  the  strip  and  the  liquid  steel  which  is  chilled  on  to  it. 

A  "shell  "or  "catch"  is  sometimes  caused  when  bottom- 
casting,  owing  to  the  steel  becoming  partly  chilled  in  the  runner 
bricks  at  the  commencement  of  teeming.  In  such  cases,  the 
steel  becomes  pasty  and  at  first  resists  the  pressure  of  the  liquid 
steel  in  the  trumpet.  As  soon  as  the  pressure  is  sufficient  to 
overcome  the  resistance  offered  by  the  pasty  steel,  the  latter  is 
suddenly  pushed  forward  into  the  mould  and  followed  by  a 
fountain  of  liquid  steel,  which  may  strike  the  side  of  the  mould 
and  leave  a  strip  of  solid  steel  adhering  to  it. 

Pulling. — Steel,  after  solidification  and  while  still  at  a  high 
temperature,  is  incapable  of  resisting  any  great  tensile  stress. 
Cooling  and  contraction  of  an  ingot  must  proceed  simultaneously, 
and  any  tendency  to  resist  this  normal  function  will  result  in  a 
"  tear  "  or  "  pull ".  An  ingot  that  becomes  fastened  to  the  top 
of  a  mould  through  flooding  or  some  other  cause  will  shrink 
away  from  the  bottom  and  then  be  forced  to  carry  its  own 
weight ;  in  such  a  case  the  tender  walls  may  at  some  point  be 
unable  to  carry  the  weight  of  the  portion  below  and  are  so  torn 
apart.  Moulds  that  for  any  reason  have  been  flooded  should 
be  immediately  cleared  from  any  "  fash,"  so  as  to  allow  free 
contraction  of  the  ingot  from  the  top  downwards.  Some  types 
of  "dozzles  "  and  such  other  rigid  devices,  if  too  firmly  fixed  to 
the  mould,  will  sometimes  hold  the  ingot  head  and  cause  pulling, 
while  the  same  thing  applies  to  "  scabbed  "  moulds  or  any  other 
cause  which  prevents  free  contraction. 

Pitting. — A  good  ingot  is  often  spoiled  by  an  inferior  skin, 
which  may  give  rise  to  small  "  rokes  "  during  the  later  stages  of 
forging.  It  is  not  uncommon  to  find  the  skin  of  an  ingot  studded 
all  over  with  small  pit  marks  ;  these  small  circular  depressions 
are  caused  by  evolution  of  gas  at  the  moment  of  contact  between 
the  steel  and  the  mould,  resulting  either  from  an  excessively  high 
casting  temperature,  or  from  iron  oxide  or  moisture  on  the 
surface  of  the  mould.  Moulds  reeked  with  tar,  which  has  not 
been  vapourised  before  teeming,  will  also  produce  the  same 
result.  Pitting  is  not  a  serious  defect,  except  in  very  small 
ingots,  and  can  be  avoided  by  careful  attention  to  casting 
temperature  and  the  use  of  clean  and  properly  reeked  moulds. 

12 


178  THE    ELECTRO-METALLURGY   OF    STEEL 

Clinks. — A  cold  ingot,  as  it  comes  from  the  casting  pit,  is 
always  under  stress  which  is  not  equally  distributed.  In  the 
case  of  certain  steels,  especially  those  belonging  to  the  air- 
hardening  class,  the  ingots  should  be  re-heated  very  slowly  in 
order  to  remove  these  unequal  stresses.  If  the  heating  is  con- 
ducted too  rapidly,  the  ingot  -may  develop  deep  seated  cracks, 
which  is  sometimes  accompanied  by  an  audible  report.  Small 
axial  cavities,  which  may  constitute  a  secondary  pipe  or  be  due  to 
internal  contraction  flaws,  may  often  serve  as  starting  points  for 
such  internal  cracks. 

Contraction  Cavities. — Messrs.  Brearley,  in  a  paper  read 
before  the  Iron  and  Steel  Institute  in  1916,  advanced  an  in- 
genious explanation  for  the  formation  of  contraction  cavities. 
Contraction  will  follow  immediately  after  solidification,  and  is 
therefore  irregular  in  different  parts  of  an  ingot ;  the  outer 
envelope  cools  first  and  can  contract  on  to  the  liquid  centre, 
until  finally  it  becomes  sufficiently  rigid  to  resist  distortion  by 
forces  exerted  upon  it  from  within.  When  solidification  is 
complete  in  the  centre  of  the  ingot,  the  outer  skin  will  resist 
the  deformation  necessary  to  reduce  the  volume  in  accordance 
with  the  internal  contraction  that  follows.  Since,  then,  the 
central  portion  of  an  ingot  must  contract,  it  can  only  do  so  by 
tearing  apart  along  a  central  axis,  where  the  steel  is  least  able 
to  resist  tensile  stress,  and  this  results  in  the  formation  of  con- 
traction cavities.  This  process  of  contraction  may  actually 
proceed  in  those  portions  that  have  just  become  solid  before 
solidification  of  the  entire  ingot  is  complete.  From  this  it 
follows  that  contraction  cavities  will  also  be  situated  at  other 
points,  particularly  where  the  steel  is  least  able  to  resist  internal 
stresses,  and  they  are  actually  found  lying  in  the  diagonal  planes 
of  weakness  which  mark  the  junction  of  chill  crystals  growing 
inwards  from  two  adjacent  faces  of  a  mould. 

Surface  Cracks. — Skin  cracks,  which  are  frequently  a  source 
of  considerable  trouble,  may  be  caused  in  several  ways.  Trans- 
verse cracks  or  tears,  produced  by  "  pulling,"  are  generally  very 
pronounced  and  always  occur  at  right  angles  to  the  direction  of 
pull ;  there  is  no  difficulty  in  recognising  this  type  of  crack, 
and  the  cause,  when  not  at  once  obvious,  is  never  difficult  to 
find.  Surface  cracks,  which  at  times  can  only  be  discovered  by 


INGOT   CASTING  179 

careful  examination,  may  result  either  from  irregular  contrac- 
tion of  the  ingot  just  after  solidification,  or  from  the  internal 
pressure  exerted  by  the  still  liquid  portion  of  an  ingot  on  the 
thin  tender  wall. 

The  formation  of  cracks  due  to  fluid  pressure  is  promoted 
by  a  high  casting  temperature  and  rapid  teeming,  as  under 
such  conditions  the  ingot  walls  towards  the  bottom  may  not 
have  attained  sufficient  thickness  to  withstand  the  pressure  due 
to  the  column  of  liquid  steel  above.  The  thin  and  tender  ingot 
walls  begin  to  contract  immediately  after  their  solidification,  so 
that  an  exceedingly  thin  gap  will  be  left  between  the  walls 
of  the  mould  and  the  newly  forming  ingot.  Internal  pressure 
may  then  cause  rupture  of  the  ingot  skin,  as  evidenced  by 
irregular  surface  cracks,  which  assume  a  more  or  less  vertical 
direction. 

Cracks  caused  by  irregular  contraction  of  the  solidified  steel 
are  due  in  some  measure  to  the  un-uniform  chilling  effect  of 
a  mould.  In  square  moulds  the  chilling  effect  is  far  greater  in 
the  corners  than  along  the  sides,  and  in  many  designs  of  moulds 
the  sides  are  cast  thicker  towards  their  centre  to  counteract 
this  effect.  Contraction  cracks  will  tend  to  develop  along  any 
natural  planes  of  weakness,  and  it  is  not  infrequent  to  find 
longitudinal  corner  cracks  which  follow  the  diagonal  plane 
marking  the  junction  of  chill  crystals.  High  casting  tempera- 
ture and  rapid  teeming  promote  this  latter  mode  of  crystallisa- 
tion and,  under  these  conditions  of  ingot  pouring,  surface  cracks 
are  almost  certain  to  be  produced  as  a  result  both  of  internal 
pressure  and  irregular  contraction. 

It  does  not  follow  that  ingots  which  show  no  apparent  sign  of 
cracks  will  forge  perfectly,  and  the  "  dry  ness  "  in  this  case  may 
probably  be  due  to  the  extension  of  small  internal  contraction 
cracks  formed  in  the  ingot  while  cooling,  especially  in  the 
neighbourhood  of  any  weak  spots,  such  as  contraction  or  shrink- 
age cavities.  In  many  cases  cracks  that  are  superficial  in  the 
ingot  will  open  out  and  extend  inwards  on  forging.  The 
defective  portion  can  be  removed  from  the  cogged  bloom  by 
chipping  or  grinding,  which,  however,  adds  considerably  to  the 
cost  of  working  down.  Frequently  a  perfectly  sound  ingot  may 
be  rendered  defective  under  the  hammer  or  press  by  improper 


180  THE   ELECTBO-METALLURGY   OF    STEEL 

reheating,  or  by  forging  at  an  unsuitable  temperature,  so  that 
before  condemning  the  original  ingot  it  is  important  to  ascertain 
whether  it  has  been  submitted  to  proper  treatment. 

Blowholes.— The  presence  of  blowholes,  distributed  irregu- 
larly in  the  body  of  an  ingot,  is  usually  due  to  the  condition  of 
the  steel  when  teeming  rather  than  to  improper  methods  of 
casting.  Blowholes  are  usually  formed  as  a  result  of  chemical 
reaction  between  dissolved  carbon  and  iron  oxide,  which  may 
be  either  present  in  the  steel  before  casting,  or  may  be  formed 
during  transfer  from  the  furnace  to  the  mould  by  pouring  over 
a  damp  spout,  or  by  using  an  imperfectly  dried  ladle  or  bottom 
pouring  trumpet.  Steels  which  are  low  in  silicon  are  more 
liable  to  be  rendered  "wild"  by  such  contact  with  moisture. 
The  evolution  of  gas,  which  occurs  internally,  is  not  observed 
until  after  solidification  has  begun.  The  oxide  being  in  a  state 
of  dilution  cannot  react  with  the  carbon  in  the  molten  steel,  but, 
on  separation  of  the  ferrite  crystals  at  the  moment  of  crystal- 
lisation, the  carbon  and  dissolved  oxides  are  brought  into  closer 
contact  by  their  resultant  concentration,  and  then  react  with 
generation  of  carbon  monoxide.  Other  theories  are  advanced 
which  account  for  the  delayed  evolution  of  gas  on  the  assump- 
tion that  the  reaction  between  the  carbides  and  oxides  only 
takes  place  at  less  elevated  temperatures. 

In  certain  cases  blowholes  may  be  caused  by  the  evolution 
of  dissolved  gases,  which  follows  as  a  result  of  their  reduced 
degree  of  solubility  at  lower  temperatures.  Excessive  over- 
heating of  steel  in  the  furnace  will  often  give  rise  to  wildness 
which  is  sometimes  impossible  to  eliminate  by  an  increased 
addition  of  silicon  and  manganese ;  it  is  difficult  to  explain  this 
behaviour,  which,  however,  has  its  analogy  in  acid  bessemer 
practice,  where  hot  blows  frequently  result  in  "  wild  heats  " 
unusually  high  in  silicon.  For  this  reason  careful  control  of 
temperature  during  the  finishing  period  of  a  heat  is  necessary, 
and  this  is  sometimes  by  no  means  an  easy  matter  with  small 
furnaces  having  a  large  power  capacity. 

Occluded  Gases. — For  many  years  past  it  has  been  thought 
that  the  presence  of  gases,  dissolved  or  occluded  in  steel,  has 
considerable  influence  on  its  physical  properties.  The  actual 
determination  of  the  quantity  of  gas  dissolved  and  its  chemical 


INGOT   CASTING  181 

composition  requires  most  elaborate  methods  of  sampling  and 
analysis,  and  even  now  no  definite  rules  governing  the  action  of 
occluded  gas  have  been  established.  At  one  time  the  addition 
of  aluminium  was  favoured,  owing  to  its  supposed  power  of  in- 
creasing the  solubility  of  gas  in  steel,  but  it  is  more  likely  that 
the  elimination  of  wildness  is  in  this  case  primarily  due  to  the 
removal  of  dissolved  oxides  and  consequent  prevention  of  their 
subsequent  reaction  with  carbon  accompanied  by  generation  of 
gas.  There  is  now  little  doubt  that  the  mere  act  of  holding 
molten  steel  in  a  tranquil  condition  prior  to  teeming  does  have 
a  beneficial  effect,  which  is  generally  accounted  for  by  the 
opportunity  afforded  of  expelling  dissolved  gas,  and  of  allowing 
suspended  slag  particles  to  rise  and  so  pass  out  of  the  steel. 
The  electric  furnace  serves  as  an  ideal  receptacle  for  applying 
such  treatment,  which  proceeds  simultaneously  with  the  final 
chemical  refining  stages  of  both  the  acid  and  basic  electric 
processes. 

Ladles,  their  Use  and  Manipulation. — Ladles  may  be  divided 
into  two  classes :  (a)  Bottom-teeming,  (6)  Lip-pouring. 

The  former  type  is  universally  adopted  for  ingot  casting,  and 
in  foundry  practice  when  basic-lined  furnaces  are  used.  The 
second  type  is  only  used  for  foundry  casting,  and  then  only 
when  it  is  an  easy  matter  to  remove  the  bulk  of  the  slag  from 
the  ladle  and  hold  back  the  remainder  while  pouring,  this  being 
only  feasible  in  the  case  of  sticky  acid  slags. 

Bottom-Teeming  Ladles. — There  are  several  types  of  bottom- 
teeming  ladles,  which  only  differ  slightly  in  mechanical  design. 
The  tilting  ladle  (Fig.  85)  consists  of  a  slightly  conical  steel 
vessel  A,  built  of  mild  steel  plates  and  mounted  on  trunnions  B, 
one  of  which  carries  a  worm  wheel  C  in  gear  with  a  worm  D.  The 
worm  is  fastened  to  one  of  two  heavy  suspension  rods  E  con- 
nected together  by  a  yoke  piece  F,  and  can  be  driven  through 
bevel  gearing  by  means  of  a  light  hand  wheel,  which  enables 
the  ladle  to  be  turned  through  a  complete  circle.  Two  small 
brackets  G  and  H  serve  as  guides  for  a  sliding  bar  I,  which  can 
be  raised  or  lowered  by  means  of  a  lever,  or  held  in  a  fixed 
position  by  a  set  screw.  This  slide  terminates  in  a  head-piece 
machined  over  its  upper  surface  to  carry  a  cross-bar  J,  the 
head  is  also  provided  with  a  screwed  pin  or  cotter  bolt,  which 


182 


THE    ELECTRO-METALLUEGY   OF    STEEL 


INGOT    CASTING 


Wedge 

Ring 


.Sleeve 


enables  the  cross-bar  to  be  firmly  secured  to  the  slide.  The 
outer  end  of  the  cross-bar  is  slotted  to  receive  the  upper  end  of 
the  stopper  rod  K,  which  in  this  case  is  provided  with  a  collar 
and  screwed  pin  or  cotter  bolt ;  the  stopper  rod  can  then  be 
firmly  fastened  to  the  cross-bar  and  so  respond  to  any  vertical 
movement  of  the  slide.  This  method  of  attaching  the  cross- 
bar to  both  the  slide  and  the  stopper  rod  allows  for  lateral  ad- 
justment of  the  latter  when  being  set  so  as  to  close  the  nozzle 
correctly.  The  lower  end  of  the  stopper  rod  (Fig.  86),  if  made 
from  a  solid  bar,  is  drilled  or  tapped  to  receive 
the  stopper  pin  ;  in  the  former  case,  the  pin  is 
slotted  and  attached  to  the  stopper  rod  by 
a  cotter.  A  hole,  through  which  the  nozzle 
brick  passes,  is  cut  in  the  bottom  plate  of 
the  ladle,  and  concentric  with  it  but  on  the 
under  side  is  fixed  the  nozzle  box,  which 
supports  the  nozzle  brick  and  fixes  its  position. 
The  walls  of  the  ladle  are  lined  from  the 
bottom  upwards  with  special  shaped  fire- 
bricks, ganister  or  damp  fire-clay  being  rammed 
to  fill  the  narrow  gap  caused  by  rivet  and 
bolt  heads.  Ladle  "  compo "  or  ganister  is 
sometimes  rammed  to  form  the  entire  lining. 
The  bottom  plate  is  covered  with  a  course  of 
fire-brick,  or  by  a  bed  of  ganister  rammed 
solid  ;  in  either  case  a  hole  is  left  a  little  larger 
in  diameter  than  the  nozzle  brick  to  be  used.  FIG.  86. 

After  lining,  the  ladle  should  be  strongly  fired 
for  twenty-four  hours  until  all  moisture  has  been  entirely  ex- 
pelled ;  failure  to  do  this  properly  may  lead  to  the  loss  of  a  cast  of 
steel.  The  stopper  rod  is  protected  by  a  covering  of  sleeve  bricks 
which  are  carefully  fitted  together,  while  the  stopper  end  (Fig. 
86)  is  firmly  attached  to  the  stopper  rod  by  a  stopper  pin.  Should 
a  slotted  pin  and  cotter  be  used  for  this  purpose,  a  rigid  con- 
nection can  only  be  secured  by  interposing  thin  washers 
between  the  stopper  brick  and  stopper  rod,  and  by  selecting  a 
suitable  cotter  from  an  assortment  of  slightly  varying  widths. 
In  the  case  of  stopper  rods  having  a  shoulder  at  the  upper  end, 
the  necessary  number  of  sleeves  must  be  threaded  on  to  it  before 


r  Cotter 
. —  Washers 
Stopper  P'm 


184  THE    ELECTEO-METALLUEGY   OF    STEEL 

fastening  the  stopper  end  brick.  The  stopper  end  and  sleeves 
are  all  mortised  to  prevent  steel  finding  its  way  through  the 
joints,  which  are  made  as  close  as  possible  with  a  little  thin 
fire-clay.  As  each  sleeve  is  joined  to  its  neighbour  below,  the 
space  surrounding  the  iron  rod  is  filled  with  sand  to  prevent 
any  lateral  movement  after  building  up.  The  sleeves  are 
usually  made  fast  by  driving  a  small  wedge  between  the  rod 
and  a  loose  washer  ring  resting  on  the  top  sleeve  brick.  Stopper 
rods  when  finished  are  sometimes  given  a  light  wash  over  with 
graphite  at  the  lower  end  and  then  slowly  but  very  thoroughly 
dried ;  any  moisture  in  the  sleeve  bricks  may  cause  them  to 
burst  when  suddenly  heated. 

The  correct  setting  of  a  stopper  rod  before  pouring  is  an 
operation  that  calls  for  considerable  care  and  skill,  and  is  usually 
only  entrusted  to  men  who  have  had  considerable  experience 
as  helpers,  and  who  have  seen  the  operation  performed  hundreds 
of  times  before  attempting  it  themselves.  The  nozzle  brick, 
after  being  placed  in  position,  is  firmly  fixed  by  ramming  loam 
or  ganister  into  the  space  round  it,  so  as  to  make  it  one  solid 
piece  with  the  rest  of  the  bottom  lining.  The  ladle  is  then 
warmed  before  setting  the  stopper  rod.  The  stopper  rod  is 
connected  to  the  cross-bar,  and  adjusted  in  the  manner  mentioned 
until  the  stopper  end  finds  a  perfect  seating  on  the  nozzle  brick 
when  lowered.  Both  the  nozzle  brick  and  stopper  end  are  con- 
vex at  their  point  of  contact,  but  owing  to  surface  irregularity 
and  slight  distortion  of  shape,  seldom  meet  exactly  to  form  a 
perfect  joint.  The  stopper  end  is  generally  ground  to  fit  the 
nozzle  before  fastening  it  to  the  stopper  rod,  this  being  done  by 
grinding  the  two  surfaces  together  with  a  little  fine  sand  before 
fixing  them  into  their  respective  positions.  This  practice  of  grind- 
ing in  with  sand  does  not  find  universal  favour,  as  it  is  argued  by 
some  that  removal  of  the  hard  skin  of  the  fire-brick  is  liable  to 
cause  trouble  after  beginning  to  teem.  After  finally  setting 
and  fastening  a  stopper  rod,  the  seating  is  tested  by  throwing  a 
little  fine,  dry,  white  sand  round  the  junction  of  the  nozzle  and 
stopper  brick ;  if,  on  lightly  tapping,  no  sand  leaks  through, 
the  stopper  rod  is  considered  sufficiently  well  set.  Sometimes 
ladlemen  purposely  set  their  stopper  rods  a  fraction  of  an  inch 
out  of  centre  towards  the  slide  bar,  so  that  the  stopper  end,  on 


INGOT   CASTING 


185 


being  lowered,  just  strikes  the  near  side  of  the  nozzle  and  then 
slides  into  the  correct  position. 

Ladles  are  always  heated  before  receiving  a  cast  of  steel,  to 
prevent  the  steel  chilling  and  stopper  troubles  that  might 
otherwise  result.  Sometimes  the  ladle  is  warmed  by  lighting 
a  small  coal  fire  on  the  bottom  after  setting  the  nozzle  brick  in 
position,  care  being  taken  to  cover  the  nozzle  brick  with  a  small 
bent  plate  to  prevent  any  ash  from  adhering  to  the  ground 
seating;  this  method  is  good  enough  for  ordinary  purposes, 
but  there  is  far  less  risk  of  skulling  and  "hard  stoppers"  if 


Blast  PipeJ 


Fi-3.  87. 


some  system  of  heating  by  air  blast  is  adopted.  Two  methods 
are  commonly  used  ;  in  one,  the  ladle  is  inverted  over  a  shallow 
open  top  fire-box  built  in  solid  masonry  (Fig.  87) ;  in  the  other 
a  small  fire  is  made  on  the  bottom  of  the  ladle,  and  a  blast  of 
air  blown  downwards  upon  it  from  a  blast  pipe  that  can  be 
readily  removed  (Fig.  88).  The  blast  pipe  A  is  constructed  in 
the  form  of  an  inverted  U,  and  is  suspended  from  a  light  jib 
that  swings  about  the  axis  of  one  limb.  This  limb  fits  loosely 
in  a  fixed  blast  pipe  B  in  which  it  is  free  to  slide  up  and  down. 
The  ladle,  containing  a  small  coke  or  coal  fire,  is  placed  close 
to  the  jib,  and  the  latter  is  then  swung  round  and  the  blast 
pipe  lowered,  so  that  the  nozzle  end  is  centrally  situated  in  the 


186 


THE    ELECTRO-METALLURGY   OF    STEEL 


ladle.  This  arrangement  is  very  convenient  as  no  extra  hand- 
ling of  the  ladle  is  required,  and  the  heat  is  produced  on  the 
bottom  where  it  is  most  wanted. 

Methods  of  Ingot  Casting. — There  are  two  entirely  distinct 
methods  of  casting  ingots,  depending  upon  whether  the  steel 
enters  the  mould  from  the  top  or  from  the  bottom.  The  former 
is  known  as  "  top-casting,"  and  the  latter  as  "  bottom-casting  " 
or  "bottom-running".  In  either  case  tapered  moulds  may 
be  used  with  their  larger  section  at  the  top  or  at  the  bottom,  as 

may  be  preferred.  Before,  how- 
ever, describing  these  different 
methods  of  casting,  the  various 
types  of  moulds  used  must  be 
considered. 

Ingot  Moulds. — Ingot  moulds 
are  of  three  kinds  (a)  open  top  and 
open  bottom ;  (6)  open  top  and 
closed  bottom ;  (c)  open  bottom 
and  closed  top. 

Moulds  should  be  made  of  a 
high  grade  grey  haematite  iron  of 
coarse  open  grain  and  high  in 
silicon  and  graphitic  carbon,  as 
the  composition  of  the  iron  used 
has  a  marked  influence  on  their 
FIG.  88.  life.  An  ingot  mould  is  always 

subjected   to  extreme  changes  of 

temperature,  more  especially  when  teeming,  and,  since  the 
heat  is  only  applied  to  the  inner  walls,  the  stresses  set  up 
by  unequal  expansion  are  considerable  ;  rough  treatment  in 
handling  also  demands  a  high  degree  of  toughness,  so  that  the 
use  of  a  No.  1  or  No.  2  grade  pig-iron  is  essential  for  the  pro- 
duction of  a  good  mould  which  will  withstand  the  severe  con- 
ditions of  service.  Moulds  up  to  20  inches  may  be  square, 
but  beyond  that  size  are  either  hexagonal  or  octagonal  in  form ; 
the  latter  shapes  favour  a  more  even  rate  of  chilling,  which 
equalises  to  some  extent  the  internal  stresses  set  up  in  the  outer 
envelope  of  the  ingot.  The  inner  walls  of  a  hexagonal  or 
octagonal  mould  are  slightly  convex,  which  allows  for  slight 


INGOT    CASTING  187 

deformation  of  the  ingot  on  cooling  and  lessens  the  possibility 
of  cracks. 

The  degree  of  taper  and  thickness  of  moulds  are  subject  to 
variation  according  to  the  particular  views  of  different  users. 
The  rate  of  chilling  or  speed  of  initial  solidification,  is  depend- 
ent upon  the  thickness  of  the  walls  and,  therefore,  the  capacity 
of  the  mould  for  absorbing  heat  from  the  liquid  steel,  so  that 
the  dimensions  of  the  mould  will  influence  the  manner  of 
crystallisation  and  also  the  position  of  any  pipe.  The  taper  of 
an  inverted  mould  with  the  larger  end  uppermost  is  often  in- 
creased so  as  to  amplify  those  conditions  which  are  favourable 
to  the  reduction  of  "  pipes  "  or  their  localisation  at  the  top  of 
the  ingot.  Closed  and  open-top  moulds  used  small  end  up 
should,  on  the  other  hand,  have  only  sufficient  taper  for  the 
purpose  of  stripping,  so  as  to  prevent  as  far  as  possible  the 
formation  of  deep-seated  pipes. 

New  moulds  often  have  a  thin  skin  of  oxide  or  dirt  on  the 
inside  faces,  and  until  they  have  received  a  few  casts  of  steel 
the  skin  of  the  ingot  will  be  somewhat  impaired  by  pitting. 
The  interior  of  the  mould  should  always  be  carefully  cleaned  with 
a  wire  scratch  brush,  and  all  scale  or  rust  removed  from  those 
which  have  been  in  disuse.  Cleaning  alone  is  not  sufficient  for 
the  production  of  a  perfect  ingot  skin,  it  being  further  necessary 
to  "reek  "  the  inner  faces  of  the  mould.  The  term  "reek" 
was  originally  used  to  denote  the  practice  of  depositing  soot  on 
the  inner  surfaces  of  small  crucible  ingot  moulds  by  exposing 
them  to  the  smoky  flame  of  burning  coal  tar  ;  now  it  is  used 
in  the  wider  sense  of  covering  or  painting  over  the  faces  with 
some  material  which  leaves  behind  a  deposit  of  carbon,  either 
on  drying  or  heating.  The  older  method  of  smoking  is  the 
best  whenever  it  can  be  used  conveniently,  but  for  large  moulds 
it  becomes  impracticable.  Two  methods  of  "  reeking "  are 
commonly  adopted  : — 

(a)  The  mould  is  brushed  over  with  a  thin  plumbago  wash 
while  it  is  warm  enough  to  drive  off  the  water.     This  method 
is  open  to  the  objection  that,  unless  the  wash  is  quite  thin,  clots 
of  plumbago  will  be  left   on  the  mould  and   produce  surface 
irregularities  and  local  unsoundness  of  the  ingot. 

(b)  The  mould  is  painted  over  with  a  thin  coating  of  tar, 


188  THE    ELECTRO-METALLURGY   OF    STEEL 

which  on  vaporising  leaves  a  thin  deposit  of  carbon  adhering 
to  the  surfaces  so  coated.  Sometimes  the  tar  is  not  boiled  off 
before  teeming,  in  which  case  there  is  a  sudden  evolution  of 
gas  at  the  moment  the  liquid  steel  comes  in  contact  with  it ; 
at  the  same  time  the  mouth  of  the  mould  will  be  filled  with 
smoke  which  prevents  the  teeming  operation  being  watched. 
When  tar  is  used  for  reeking  it  is  better  to  clean  the  mould 
while  still  hot  from  a  previous  cast,  and  then  to  paint  it  over 
so  that  all  the  volatile  matter  may  be  driven  off  before  it  is  again 
required  for  use. 

Ingot  Pit. — Moulds  are  usually  set  in  a  specially  prepared 
pit,  so  that  their  upper  ends  are  at  a  convenient  height  above  the 
shop  floor  level  for  teeming.  Sometimes  the  moulds  are  set  on 
the  floor,  in  which  case  it  is  necessary  to  provide  a  platform 
from  which  the  ladleman  can  manipulate  the  stopper  rod  lever. 
Ingot  pits  are  preferable  from  every  point  of  view,  and  should 
always  be  provided  wherever  possible.  When  moulds  are  set 
on  the  floor  level  there  is  a  far  greater  risk  of  injury  to  workmen 
in  the  event  of  a  running  stopper  or  a  break  out  at  the  bottom 
of  a  mould  or  trumpet,  besides  which  the  manipulation  of  the 
ladle  in  the  event  of  a  hard  stopper  is  rendered  far  more  diffi- 
cult and  dangerous.  Ingot  pits  are  usually  lined  with  fire-brick, 
and  the  walls  slightly  tapered  and  crowned  with  a  cast-iron  curb 
plate  to  facilitate  the  withdrawal  of  any  steel  skull  that  may 
have  accidentally  covered  the  bottom  from  side  to  side. 

Top- Casting. — The  method  of  filling  an  ingot  mould  from  the 
top  is  by  far  the  most  commonly  adopted,  and  is  almost  invari- 
ably used  for  casting  ingots  heavier  than  about  30  cwts.  Some- 
times split  moulds  are  used  which  have  no  longitudinal  taper 
and  form  a  closed  bottom.  Moulds  that  are  not  split  must  be 
tapered  longitudinally  to  ensure  ease  of  stripping,  and  are  used 
with  either  the  small  or  big  end  uppermost.  Ingots  that  have 
been  cast  from  the  same  mould  but  in  reversed  positions  will 
not  have  similar  internal  structures,  so  that  when  selecting  a 
method  of  casting,  not  only  should  the  relative  ease  of  handling 
the  moulds  be  considered,  but  also  the  probable  position  and 
effect  of  the  pipe  produced. 

As  regards  handling,  it  is  clear  that  moulds  set  small  end  up 
will  have  to  be  stripped  from  off  the  ingots,  and  then  reset  in 


INGOT   CASTING 


189 


position  for  a  subsequent  cast ;  this  does  not  apply  in  the  case  of 
ingots  cast  large  end  up,  which  can  be  removed  from  the  moulds 
without  disturbing  their  position,  provided  small  wrought-iron 
eyes  are  fixed  in  the  head  of  the  ingots  just  before  setting. 
Apart  from  the  question  of  handling,  the  direction  of  taper 
exerts  a  considerable  influence  on  the  position  and  magnitude 
of  a  pipe,  which  may  be  explained  by  reference  to  Figs.  89  and 
90.  Fig.  89,  which  shows  the  section  through  an  ingot  cast 
small  end  up,  indicates  a  pipe  extending  half  way  down  together 
with  a  small  secondary  pipe.  On  studying  the  progress  of 
solidification  in  such  a  mould  it  is  clearly  apparent  that  the 
chilling  effect  at  the  head  of  the  mould  is 
greater  than  at  lower  sections  owing  to 
its  contracted  area,  the  rapid  loss  of  heat 
from  the  liquid  steel  exposed,  and  the 
sometimes  increased  thickness  of  the 
mould  walls  towards  the  top.  The  top 
end  of  an  ingot  may  then  solidify  almost 
as  rapidly  as  the  bottom,  so  that  the 
shrinkage,  which  accompanies  solidifica- 
tion in  the  lower  portion  of  the  ingot, 
cannot  be  fed  sufficiently  by  liquid  steel 
from  above ;  this  results  in  the  formation 
of  an  extended  cavity  or  pipe. 

Clearly,  any  modification  of  condi- 
tions which  will  delay  the  solidification  of 
the  upper  portion  of  an  ingot  relatively  to  the  lower,  will  not 
only  alter  the  ultimate  shape  of  the  cavity,  but  will  also 
eliminate  the  extended  portion  together  with  the  secondary 
"pipes".  Such  conditions  are  better  fulfilled  by  reversing 
the  direction  of  taper,  when  the  shape  of  the  pipes  formed 
will  resemble  that  shown  by  Fig.  90.  The  solidification  of 
the  upper  portion  of  an  ingot  may  be  further  retarded  by  in- 
creasing the  taper  of  an  inverted  mould,  so  that  in  this  case 
taper  can  be  used  as  a  beneficial  influence  within  limits,  whereas 
for  moulds  used  small  end  up  the  result  would  be  still  more 
disastrous.  Moulds,  specially  made  for  casting  large  end  up, 
are  either  provided  with  a  solid  bottom  or  must  be  machined  all 
over  the  lower  open  end,  so  that  perfect  contact  with  a  flat 


FIG.  89.     Fio.  90. 


190  THE    ELECTRO-METALLUEGY   OF    STEEL 

surfaced  chill  plate  may  be  ensured.  Any  gap  between  the 
mould  and  chill  plate  would  immediately  be  filled  with  steel  when 
teeming,  which  would  prevent  the  ingot  being  stripped  without 
the  added  labour  and  expense  of  removing  the  "  fash  ".  Moulds 
of  this  type  are  often  cast  with  their  outside  faces  parallel,  so 
that  the  thickness  of  the  walls  at  the  bottom  end  is  greater  than 
at  the  top  ;  this  still  further  tends  to  accelerate  solidification  of 
the  bottom  portion.  So  far,  then,  as  the  use  of  moulds  is  con- 
cerned, those  conditions  which  tend  to  reduce  the  extent  of 
piping  are  best  satisfied  by  top-casting  into  moulds,  large  end 
uppermost,  when  the  effect  of  wall  thickness  and  degree  of  taper 
may  also  be  used  to  good  advantage.  A  closed-bottom  inverted 
mould  is  usually  dished  at  the  bottom,  and,  provided  care  is 
taken  when  teeming,  its  life  may  be  as  long  as  that  of  an  open- 
bottom  mould.  An  inexperienced  teenier  can,  however,  easily 
ruin  a  mould  by  opening  out  the  nozzle  too  sharply  at  the  start, 
and  so  burning  the  bottom  to  such  an  extent  that  "  stickers  " 
will  result  in  subsequent  casts  ;  this  applies  especially  when 
casting  high  carbon  steels. 

The  bottom  or  "  chill  "  plate  used  with  open-bottom  inverted 
moulds  are  cast  with  a  deep  concave  depression  in  the  centre, 
so  that  the  splash  caused  by  the  stream  striking  the  bottom  is 
prevented  from  touching  the  walls  of  the  mould,  and  from 
penetrating  any  open  joint  between  the  mould  and  the  bottom 
plate.  The  depression  in  the  chill  plate  is  also  rapidly  filled 
with  steel,  which  then  breaks  the  force  of  the  stream  and  so 
further  prevents  splashing. 

Bottom -Casting. — This  method  is  one  that  is  most  usually 
and  conveniently  used  for  casting  a  large  number  of  ingots  of  small 
dimensions.  If  a  large  number  of  small  ingots  have  to  be  cast 
separately  from  a  ladle,  there  is  every  possibility  that,  owing  to  the 
very  slow  rate  of  teeming  necessary,  the  steel  will  become  chilled 
and  too  cold  before  the  moulds  are  all  filled.  By  adopting  a  system 
of  bottom-casting  a  large  number  of  moulds  may  be  filled 
simultaneously,  so  that  the  rate  of  filling  each  mould  is  ex- 
tremely slow  compared  to  the  actual  rate  of  teeming  ;  the  ladle 
is  thus  emptied  in  a  far  shorter  time  and  the  proper  condition 
of  slowly  filling  each  mould  is  satisfied.  Closed-top  moulds, 
having  only  a  small  conical  gas  vent,  are  frequently  used,  and 


INGOT   CASTING 


191 


have  certain  advantages  over  the  open-top  moulds.     A  group  of 
such  moulds  is  shown  in 
Fig.   91,   where  they   are 
mounted     on     a     bottom 
runner  plate   suitable   for 
casting  four  ingots.     The 
bottom    plate    is    usually 
cast  with  four  or  six  lateral 
recesses  radiating  from  a 
central    recess,    which   is 
accordingly  either  square 
or  hexagonal.    The  central 
recess   contains   a    centre 
brick  which  is  simply  fitted 
within  it  by  a  packing  of 
dry  sand,  while  the  lateral 
recesses   are    suitably   di- 
mensioned   to    hold     the 
runner-bricks,    which    are 
mortised  to  fit  into   each 
face  of   the   centre  brick. 
If  the  centre  of  the  mould 
is    far    from    the    centre 
of  the  plate  an  extension 
runner-brick    of     suitable 
length  is   used.      Fig.  92 
shows  a  four-way  and  six- 
way  centre   brick.     Both 
the  centre  and  the  runner- 
bricks  must   be   carefully 
set  in  the   recessed   plate 
with    sand,    so   as    to   be 
flush   with  the  top  of  it. 
The  mould,  or  at  least  one 
side  of  it,  will  rest  across 

the    brick,    so    that    any  ^ 

difference  in  level  between  FIG.  91. 

the   plate   and    the   brick 

may  cause  the  steel  to  run  out  and  the  ingots  to  "  bleed".     The 


192 


THE    ELECTRO-METALLURQY   OF    STEEL 


runner-brick  is  closed  at  one  end,  but  is  provided  with  an  orifice 
that  is  best  surrounded  by  an  annular  flange,  which  helps  to  de- 
flect the  stream  of  steel  vertically  upwards.  It  is  most  important 
that  the  moulds  are  set  on  the  plate  exactly  central  with  this 
orifice,  otherwise  the  side  of  the  mould  nearest  the  stream  of  steel 
may  be  washed  and  burnt ;  careless  setting  may  also  cause  cracked 
ingots,  due  to  the  unequal  distribution  of  hot  steel  in  the  mould 
(Brearley).  The  steel  is  fed  into  the  centre  brick  from  a  vertical 
fire-brick  pipe,  enclosed  in  a  cast-iron  or  steel  frame  called  a 
''trumpet".  Fig.  91  shows  a  section  through  such  a  trumpet, 
each  half  of  which  is  built  up  of  semicylindrical  sections  fastened 
together  by  bolts  or  cotter  pins  ;  in  the  figure  it  is  shown  in  one 
length,  the  two  halves  being  cottered  together.  The  pipe  bricks, 


0:4      t-WQj 


FIG.  92. 


FIG.  93. 


FIG.  94. 


(Fig.  93)  are  carefully  laid  and  fitted  together  in  one  half  of 
the  trumpet  on  a  bed  of  "  compo  "  or  ganister,  the  bottom  brick, 
which  is  sometimes  mortised  at  its  lower  end  to  fit  the  centre 
brick,  being  exactly  flush  with  the  end  of  the  trumpet.  The 
bell  section  is  finally  set  in  position,  and  the  entire  pipe  daubed 
over  with  ganister  or  "  compo".  The  other  half  of  the  trumpet 
is  then  laid  upon  it  and  cottered  to  the  under  half,  squeezing  out 
any  excess  of  the  bedding  material.  The  built-up  trumpet  is 
carefully  dried  before  use  to  prevent  the  brick  pipe  bursting 
through  sudden  generation  of  steam.  The  lowest  section  or 
bottom  end  of  the  trumpet  has  a  flange,  which  is  either  bolted 
on  to  the  bottom  plate  or  held  down  by  weights,  as  shown  in 
Fig.  91 ;  this  is  done  to  prevent  the  steel  finding  its  way  under 


INGOT   CASTING  193 

the  trumpet  and  lifting  it  owing  to  the  pressure  of  the  column 
of  steel  in  the  trumpet  pipe. 

When  closed-top  moulds  are  used,  the  steel  rises  up  until  it 
reaches  the  base  of  the  vent,  where  it  then  normally  chills,  but 
if  teeming  at  this  point  is  too  rapid  and  the  steel  very  hot  it 
may  sometimes  spurt  out;  any  "fash"  so  formed  round  the 
vent  hole  is  immediately  removed  from  the  mould  top  to  pre- 
vent the  steel  being  held  fast  in  the  vent.  Should  the  latter 
frequently  occur  the  vent  becomes  worn  and  enlarged,  causing 
constant  trouble  with  "  stickers  ".  A  plan  commonly  adopted 
is  to  build  a  small  mound  of  loam  above  the  vent  hole  and  then 
prick  through  with  a  large  nail,  in  this  way  the  conical  plug  of 
steel  filling  the  vent  cannot  become  fastened  to  the  mould  top. 
Care  is  always  taken  to  dry  the  runner  bricks  and  moulds 
thoroughly,  as,  apart  from  the  possibility  of  the  bricks  bursting, 
there  is  sometimes  a  danger  of  the  moulds  lifting  through  any 
sudden  generation  of  steam.  After  the  steel  has  become  solid 
in  the  vent,  the  ingots  may  be  constantly  fed  under  pressure 
from  the  steel  in  the  trumpet,  since  the  bell  will  be  at  least 
12  inches  above  the  ingot  top.  In  the  case  of  open-top  moulds, 
the  bell  will  be  at  the  same  level  as  the  ingot  head,  and  con- 
sequently no  feeding  is  possible  from  the  trumpet ;  "dozzles  " 
and  "  cheek  "  bricks  may  be  fitted,  however,  in  the  top  of  the 
mould  to  delay  solidification  of  the  ingot  head,  and  so  reduce 
the  extent  of  "  piping  ". 

The  solidification  of  a  bottom-cast  ingot  takes  place  in  a 
different  manner  from  one  top-cast.  In  the  latter  case  hot 
steel  is  constantly  fed  from  the  top,  so  that,  at  the  moment  of 
filling,  the  steel  in  the  head  of  the  mould  will  be  hotter  than 
elsewhere ;  in  the  former  case  these  conditions  are  reversed, 
so  that  the  region  of  piping  in  the  ingot  will  be  lowered, 
following  upon  the  slower  cooling  of  the  lower  portion  of  the 
ingot  relatively  to  the  top ;  at  the  same  time,  the  chilling  effect 
at  the  head  of  the  mould  is  greater  than  at  the  bottom,  which 
further  accentuates  the  tendency  towards  the  formation  of  an 
extended  primary  pipe  and  small  secondary  pipes.  This 
method  of  bottom-casting  small  ingots  in  groups  may  be  safely 
adopted,  provided  there  is  no  danger  to  be  anticipated  from  the 
presence  of  pipes,  which  are  always  most  pronounced. 

13 


194  THE    ELECTEO-METALLUEGY   OF    STEEL 

There  is  yet  one  other  modification  of  the  method  of  bottom- 
casting,  which  entails  the  use  of  a  special  mould,  having  a 
partially  closed  bottom  and  used  large  end  uppermost.  Such 
an  arrangement  is  shown  by  Fig.  94.  The  bottom  of  the  mould 
is  thick,  and  has  a  central  conical  hole  into  which  fits  a  fire- 
brick plug  designed  to  make  a  morticed  connection  with  the  orifice 
of  the  runner-brick.  The  stream  of  steel  is  guided  vertically 
upwards  without  risk  of  washing  the  sides  of  the  mould.  The 
bottom  of  the  ingot  is  exposed  to  a  greater  chilling  effect,  and, 
since  the  upper  portion  of  the  ingot  is  wider,  the  rate  of 
solidification  is  there  retarded.  These  altered  conditions  in 
the  process  of  solidification  tend  to  reduce  the  length  of  pipe 
and  eliminate  secondary  pipes,  so  that  the  arrangement  pre- 
sents distinct  advantages  over  other  methods  of  bottom-casting. 
There  is,  however,  one  serious  objection,  which  lies  in  the 
preparation  and  correct  setting  of  the  mould  before  casting.  It 
is  obvious  that  any  open  joint  between  the  fire-brick  plug  and 
the  mould  will  become  filled  with  steel  and  prevent  withdrawal 
of  the  ingot ;  unfortunately,  it  is  not  always  possible  to  rely 
upon  this  joint  being  properly  filled  up  with  clay,  so  that  the 
success  of  the  method  will  depend  entirely  upon  the  human 
element.  All  methods  of  bottom-casting  demand  considerable 
care  from  pitmen,  as  any  loose  dirt  left  in  the  runner-bricks 
will  be  washed  away  and  probably  become  entrapped  in  the 
ingot. 

Dozzles,  Cheek = bricks,  and  Sinking  Head. — It  was  realised  in 
the  early  days  of  crucible  ingot  casting  that  the  formation  of  a 
pipe  might  be  prevented  by  keeping  the  head  of  an  ingot  molten 
until  solidification  was  complete  at  lower  levels.  This  was 
accomplished  by  placing  a  strongly  heated  fire-brick,  with  a 
slightly  conical  central  hole,  in  the  head  of  the  mould  while 
teeming  was  momentarily  stopped ;  the  steel  quickly  solidifies 
round  its  lower  edge  and  holds  it  fast  to  the  ingot  walls.  The 
central  cavity  in  the  brick  is  then  filled  with  steel,  which  remains 
molten  and  serves  as  a  reservoir  from  which  liquid  steel  is  con- 
stantly drained  as  shrinkage  proceeds  lower  down.  Such  bricks 
are  usually  known  as  "dozzles"  or  "cores,"  and  are  made  in 
a  large  variety  of  shapes  and  sizes  (Fig.  95).  It  is  noti  always 
possible  to  pre-heat  the  largest  sizes,  and  the  beneficial  effect  is 


INGOT   CASTING 


195 


then  greatly  lessened.  "  Cheek  ' '  bricks  are  used  for  large  moulds 
in  place  of  the  fire-clay  "  dozzle,"  and  are  moulded  with  a  small 
lug  on  their  upper  edge  which  rests  on  the  top  of  the  mould 
(Fig.  96).  As  these  bricks  may  sometimes  become  firmly 
attached  to  the  ingot  head,  the  lugs  should  be  cracked  after 
teeming  to  prevent  any  tendency  to  hold  the  ingot  and  cause  it 
to  "pull". 

"Sinking  heads"  are  usually  employed  for  large  ingots, 
and  consist  of  a  light  cast-iron  or  steel  box,  rammed  up  with 
loam,  "compo,"  or  moulding  sand,  leaving  a  central  cavity 
shaped  and  tapered  as  desired.  The  "sinking  head  "rests  on 
the  mould  top,  and  care  must  be  taken,  when  setting  in  posi- 
tion, that  the  joint  is  well  closed  to  prevent  "fashes"  being 
formed  which  will  hold  the  ingot  and  cause  it  to  pull.  Some- 
times the  upper  part  of  a  large  mould  is  recessed  all  round,  and 


FIG.  95. 


FIG.  96. 


the  space  filled  up  with  "  compo  "  or  other  material;  in  this 
way  the  sinking  head  becomes  part  of  the  mould  itself.  In 
many  cases  the  use  of  such  devices  is  not  alone  sufficient,  so 
that  charcoal  is  sometimes  thrown  on  the  liquid  steel  to  further 
retard  solidification  by  the  heat  of  its  combustion.  Sir  Eobert 
Hadfield  advocated  the  use  of  a  layer  of  fusible  neutral  slag 
between  the  steel  and  the  charcoal,  and  also  used  an  air  blast  to 
promote  a  higher  temperature  of  combustion.  With  this  pro- 
cess the  steel  sinks  quite  level  in  the  "  compo  "  or  ganister-lined 
sinking  head,  leaving  only  a  shell  of  steel  adhering  to  the  sides. 
Tun = dish  Casting. — The  importance  attached  to  the  speed 
of  teeming  cannot  be  exaggerated,  while  the  difficulty  of  casting 
a  large  number  of  small  ingots  under  nearly  similar  conditions 
from  a  large  ladle  has  been  already  mentioned.  Group  casting 
certainly  provides  one  solution  of  the  difficulty,  but  adds  to  the 


196 


THE    ELECTEO-METALLURGY   OF    STEEL 


cost  of  casting,  and  does  not  produce  such  a  reliable  ingot  as  one 
top-cast  large  end  up. 

It  is  only  a  few  years  since  it  was  proposed  to  interpose  an 
auxiliary  receptacle  oetween  the  ladle  and  the  moulds  for  the 
purpose  of  dividing  the  stream  from  the  ladle  into  several 
smaller  streams  that  feed  directly  into  the  moulds.  Such  a  re- 
ceptacle is  known  as  a  tun-dish,  an  example  of  which  is  shown 
in  Fig.  97.  The  advantages  of  this  system  of  casting  have  been 
well  proved,  and  one  instance  may  be  quoted  where  the  quantity 
of  billets  regularly  scrapped  on  account  of  cracks  was  reduced 
from  over  50  per  cent,  down  to  5  per  cent,  and  under.  J.  N. 
Kilby  has  given  some  conclusive  figures  proving  the  value  of 
the  tun-dish,  which  may  be  used  even  in  large  plants  without 

causing  any  inconvenience  in  the 
shop.  A  separate  tun-dish  is  usually 
supported  above  each  group  of 
moulds  that  it  feeds,  but  sometimes 
it  is  attached  to  the  under-side  of 
the  ladle  and  moves  with  it.  They 
are  generally  designed  to  feed  two 
moulds,  but  are  also  used  for  four. 
The  tun-dish  must  itself  be  carefully 
fed,  so  that  the  stream  of  steel 
from  the  ladle  is  equally  divided 

between  the  several  nozzles.  Failing  to  do  this,  the  moulds 
will  not  fill  at  the  same  speed  and  ingots  of  unequal  length  will 
result,  which  may  prove  serious  in  cases  where  the  moiilds  are 
provided  with  dozzles  or  cheek-bricks.  The  dish  is  lined  with 
either  thin  fire-brick  splits,  or  a  facing  of  rammed  "  compo,"  or 
ganister,  and  is  usually  deep  enough  to  hold  a  reservoir  of  steel 
at  least  8  inches  in  depth,  which  helps  to  equalise  the  pressure 
of  steel  above  each  nozzle,  and,  at  the  same  time,  prevents  the 
stream  of  steel  from  the  ladle  splashing  up  from  the  bottom. 
Apart  from  the  slow  speed  of  filling,  a  further  advantage  is 
gained,  since  the  streams  from  the  tun-dish  are  steady  and  not 
forced  through  the  nozzles  under  great  pressure  ;  the  commotion 
in  the  mould  is  therefore  far  less,  and  the  skin  of  the  ingot  freer 
from  the  usual  splash  markings  of  top  cast  ingots. 


FIG.  97. 


CHAPTEE  X. 

APPLICATION  OF  THE  ELECTRIC  FUENACE  TO  FOUNDRY  PRACTICE. 

Introduction. — The  electric  furnace  has  certain  features  that 
make  it  especially  suitable,  and  in  many  respects  superior  to 
other  steel-making  plant,  for  the  manufacture  of  light  steel 
castings.  It  has,  of  course,  disadvantages  as  well  as  advantages, 
and  it  is  only  by  carefully  studying  the  operating  conditions,  both 
technical  and  economic,  that  its  suitability  may  be  determined 
for  individual  cases.  Prior  to  the  introduction  of  electric  melting 
into  foundry  practice,  castings  of  thin  section  and  high  ductility 
were  made  by  the  crucible  process,  or  by  employing  some  form 
of  small  converter  such  as  the  Tropenas,  Bessemer,  and  later, 
the  Stock.  The  latter  plants  are  satisfactory,  provided  that 
steel  with  a  carbon  content  of  less  than  *2  per  cent,  to  '25  per 
cent,  is  not  regularly  required  for  castings  of  very  thin  section. 
There  is  no  doubt  that  thin  castings  with  a  carbon  content  as 
low  as  *1  per  cent,  to  '15  per  cent,  can  be  made,  at  any  rate, 
from  the  Stock  converter,  but  such  blows  cannot  be  repeated 
with  absolute  certainty  under  the  always  variable  conditions 
of  blowing. 

With  the  rapid  development  of  motor  engineering  and  the 
introduction  of  aeroplane  construction,  there  has  been  a  growing 
demand  for  steel  castings  of  thin  section,  but  not  necessarily  of 
small  dimensions.  The  factor  of  safety  is  an  element  that  has 
to  be  most  carefully  considered  in  these  particular  branches  of 
the  engineering  industry,  so  that -the  demand  for  light  castings 
made  from  steel  of  a  high  degree  of  purity  and  having  the 
necessary  reliability  in  service  has  now  become  general. 

The  chemical  composition  of  crucible  steel  is  dependent 
upon  the  quality  of  scrap  melted,  and  the  degree  of  carbon  absorp- 
tion from  the  pot.  Clay  pots  are  not  so  satisfactory  for  foundry 
purposes  as  the  tougher  and  more  refractory  graphite  crucibles, 

(197) 


198  THE   ELECTRO-METALLURGY   OF   STEEL 

which  may  be  allowed  to  cool  off  between  melts.  Apart  from 
the  high  cost  of  the  latter  type,  there  is  the  disadvantage  that 
unless  special  precaution  is  taken  to  add  a  little  ore  to  the 
charge,  the  absorption  of  carbon  by  the  steel  will  prevent  a  high 
degree  of  ductility  being  obtained.  The  iron  ore  is  added  in 
slight  excess  of  the  quantity  required  to  boil  out  the  carbon 
absorbed,  so  that  the  steel,  when  hot  enough  for  pouring,  will 
be  oxidised  and  require  somewhat  drastic  "killing"  with 
aluminium  before  casting  into  moulds.  It  must  not  be  as- 
sumed that  castings  made  in  this  way  from  crucible  steel  are 
not  of  excellent  quality,  provided  the  raw  materials  are  good, 
and,  for  a  very  small  output  of  uniformly  light  castings,  of  which 
the  heaviest  can  be  poured  from  one  or  at  the  most  two  pots, 
there  is  a  good  deal  to  be  said  in  favour  of  this  method  of  steel 
melting.  The  production  of  mild  steel  from  crucibles  is  almost 
entirely  a  matter  of  steel  melting,  and  can  hardly  be  expressed 
as  "steel-making".  When  a  crucible  charge  is  melted  and 
hot  enough  to  pour,  no  attempt  can  be.  made  to  control  the 
carbon  content,  and  it  is  only  on  examination  of  a  casting  itself 
that  the  suitability  of  the  steel  can  be  judged.  When  dead 
soft  steel  is  melted  in  graphite  crucibles,  the  final  carbon  will 
in  most  cases  reach  '2  per  cent,  and  at  times  will  fall  within  a 
wider  range,  reaching  up  to  '6  per  cent.  Such  cases  as  the  latter 
are  not,  of  course,  frequent,  but  are  mentioned  merely  to  indi- 
cate that  the  crucible  product  is  not  under  the  same  degree  of 
control  as  converter  or  electric  furnace  steel. 

Prior  to  1914  the  converter  at  least  held  its  own  for  foundry 
purposes,  but  the  subsequent  shortage  of  suitable  raw  material, 
and  the  increasing  confidence  in  the  economic  performance  of 
electric  furnaces,  caused  the  latter  to  gain  favour  for  the  manu- 
facture of  light  soft  castings.  The  converter  is  restricted  to  the 
acid  process,  and  consequently  the  composition  of  the  final  steel 
as  regards  sulphur  and  phosphorus  is  dependent  solely  upon  the 
composition  of  the  raw  material  used.  The  temperature  of 
casting  will  likewise  depend  upon  the  silicon  content  of  the 
charge,  which  must  necessarily  be  high  if  a  high  temperature 
and  degree  of  fluidity  is  required ;  this  imposes  a  limitation  to 
the  percentage  of  scrap  in  a  charge,  and  to  the  quality  of  the 
pig-iron  used.  The  acid  process  is  well  suited  for  foundry 


APPLICATION  OF  ELECTKIC  FURNACE  TO  FOUNDRY  PRACTICE      199 

purposes,  since  the  character  of  the  slag  renders  its  separation 
from  the  steel  a  matter  of  great  ease,  and  therefore  allows  the 
system  of  lip-pouring  to  be  adopted.  The  latter  method 
certainly  has  advantages  over  bottom-teeming,  which  is 
more  likely  to  cause  "  scabbing  "  and  dirty  castings,  but,  on 
the  other  hand,  a  higher  casting  temperature  is  necessary. 
Stopper  troubles  disappear  and  the  steel  always  enters  the 
mould  at  a  steady,  low  pressure,  due  solely  to  the  fall  from  the 
ladle  lip  and  not  to  the  pressure  exerted  by  a  head  of  steel  be- 
hind the  stream,  as  in  bottom  teeming.  On  the  other  hand, 
unless  handshanks  are  used,  lip- pouring  is  not  suitable  for  very 
light  work  which  requires  a  precise  and  rapid  ladle  control. 
The  limitation  of  the  converter  to  the  acid  process  is,  so  far  as 
the  problem  of  casting  is  concerned,  an  advantage  rather  than 
otherwise,  as  both  lip-pouring  and  bottom-teeming  are  open  to 
choice  according  to  the  nature  of  the  work  being  cast. 

The  electric  furnace  offers  far  more  scope  to  the  steel-maker 
than  the  converter,  and  from  a  purely  technical  standpoint,  the 
quality  of  the  steel  is  almost  independent  of  the  class  of  steel 
scrap  used  for  the  furnace  charge.  It  is  optional  to  use  either 
the  basic  or  acid  process,  and  the  ultimate  choice  will  depend 
upon  the  specification  demanded  and  the  quality  of  scrap  avail- 
able. The  temperature  of  casting  is  only  limited  by  the  fusing 
point  of  refractories,  and  the  chemical  composition  of  the  steel 
is  always  under  exact  control.  The  relative  advantages  of  the 
basic  and  acid  processes,  as  conducted  in  the  electric  furnace, 
require  further  consideration. 

The  basic  process  enables  both  phosphorus  and  sulphur  to 
be  reduced  to  very  low  limits,  so  that  any  chemical  specification 
can  be  readily  met,  even  if  impure  scrap  high  in  both  these 
elements  is  used.  The  deoxidation  of  the  steel  can  also  be  con- 
ducted under  highly  reducing  conditions,  which  certainly  presents 
great  advantages  over  the  cruder  method  of  "  killing "  wild 
steel  by  alloy  or  aluminium  additions  immediately  before  cast- 
ing. In  cases  where  the  amount  of  phosphorus  and  sulphur 
present  in  the  raw  material  exceeds  the  specification  limit  of 
the  casting,  the  basic  process  becomes  obligatory.  On  the 
other  hand,  if  the  scrap  available  is  sufficiently  low  in  sulphur 
and  phosphorus,  and  merely  requires  remelting  with  adjustment 


200  THE   ELECTRO-METALLURGY   OF    STEEL 

of  carbon,  silicon,  and  manganese,  then  the  acid  process,  so  far 
as  the  actual  steel-making  is  concerned,  will  fulfil  the  purpose. 
If,  then,  it  is  not  imperative  to  use  the  basic  process,  the  choice 
of  process  will  rest  solely  upon  economic  considerations  based 
upon  the  prices  of  what  may  be  called  low  grade  and  high 
grade  scrap,  and  other  minor  advantages  which  each  may 
present.  The  chemistry  of  steel-making  with  an  acid  slag  is 
practically  the  same  for  both  open  hearth  and  electric  furnaces, 
when  applied  to  foundry  practice,  only  in  the  latter  case  the 
entire  or  predominant  portion  of  the  charge  will  be  steel  scrap, 
which  renders  the  possibility  of  over-oxidation  of  the  bath  more 
considerable.  For  this  reason,  the  scrap  should  be  selected  and 
be  free  from  rust,  since  the  process  of  deoxidation  cannot  be 
carried  to  the  same  extent  as  under  a  basic  slag.  The  acid 
process  certainly  shows  a  small  saving  in  power  consumption 
since  the  extra  time  occasioned  by  the  use  of  a  second  slag  is 
eliminated.  The  steel  may  be  finished  and  poured  when  the 
correct  casting  temperature  has  been  reached,  assuming  that  the 
bath  has  been  properly  boiled  down  and  is  thus  fairly  free  from 
oxides.  The  convenience  of  handling  steel  under  an  acid  .slag 
has  been  already  mentioned,  and  should  not  be  disregarded 
when  deciding  upon  the  process  to  be  adopted.  For  cleanliness 
and  convenience  of  operation  the  acid  process  is  to  be  preferred, 
and  will  be  found  rather  more  economical,  if  scrap  of  suitable 
quality  and  price  can  be  secured. 

The  comparative  advantages  and  disadvantages  of  the 
crucible,  converter,  and  electric  processes  for  the  manufacture  of 
light  steel  castings  are  summarised  and  tabulated  in  Table  I. 

Early  Development  and  Statistics. — It  was  early  recognised 
that  the  electric  furnace  offered  special  advantages  in  certain 
departments  of  foundry  practice  on  account  of  the  high  casting 
temperature  attainable.  The  Stassano  furnace  was  at  first 
favoured  for  this  purpose,  but  chiefly  owing  to  the  severe  punish- 
ment of  the  magnesite  roof,  furnaces  of  the  direct  arc  type 
are  now  more  widely  used.  In  the  year  1909  the  firm  of  George 
Fisher  in  Switzerland  had  installed  a  small,  single-phase 
Heroult  furnace  for  the  production  of  very  light  steel  castings, 
which  included  the  first  type  of  hollow-spoke  motor  vehicle 
wheels  used  in  Great  Britain.  The  first  electric  furnace  laid 


APPLICATION  OF  ELECTRIC  FURNACE  TO  FOUNDRY  PRACTICE     "201 


TABLE  I. — COMPARISON  OF  THE  CRUCIBLE,  CONVERTER,  AND  ELECTRIC  FURNACE  PROCESSES 

FOR  FOUNDRY  USE. 


Crucible. 

Side-blown  Converter. 

Electric  Furnace. 

Character  of 

100  per  cent,  wrought- 

It  is  possible  to   blow  a 

A    charge     entirely    com- 

the      raw 

iron    or    steel    scrap 

charge     containing     30 

posed   of   scrap  can   be 

materials 

commonly  used. 

per    cent,     of    foundry 

used.     All   the   foundry 

used,   and 

Small    proportion   of 

scrap,  p.ovided   a  high 

scrap  made  can   be  re- 

proportion 

clean   foundry    scrap 

silicon  haematite  pig  is 

melted,  if  desired.     Usu- 

of     scrap 

also  melted  in  charge. 

used,  but  20  per  cent,  is 

ally    the    foundry   scrap 

melted. 

Impossible  to  re-melt 

a  more  usual  figure.  Can 

made     is     worth    more 

all  the  foundry  scrap 

use    up     the     bulk     of 

than     the     steel    scrap 

made.         Value      of 

foundry     scrap      made. 

bought   for  melting,    in 

foundry     scrap     less 

Value    of    the    foundry 

which  case  it  is  sold  at 

than  the  raw  mater- 

scrap  is   less  than  the 

a  figure  that  has  appreci- 

ials used. 

average     value    of    the 

ated  by  treatment  in  the 

metal  before  blowing. 

furnace. 

Melting 

Very  small,  and  depen- 

Usually a  14  per  cent,  to 

Loss    averages     about    7 

loss. 

dent  upon  the  cleanli- 

24 per  cent,  loss  on  con- 

per cent,  to  10  per  cent., 

ness    of    the     scrap 

version  of  the  clean  raw 

when  using  fairly  clean 

melted.       Loss     en- 

materials  consisting  of 

scrap  in  a  form  that  is 

tirely  chemical. 

pig    and     steel     scrap. 

not    liable  to   excessive 

Loss  is  both  mechanical 

oxidation  in  the  furnace. 

and  chemical. 

Loss    both    mechanical 

and   chemical   in   basic 

furnaces,  and   chemical 

only  in  acid  furnaces. 

Fuel  cost. 

Coke  melting  expensive 

Cost   of  coke   or  oil  fuel 

The   cost    of    melting    is 

and   consumption   of 

used  for  melting  the  raw 

high   and  depends  also 

fuel  high  under  forced 

material    is    very    low. 

upon   the   average   load 

draught.     Gas,  when 

Fuel  cost   per  ton  steel 

factor  or  output  over  a 

used,      is      cheaper. 

blown    is  almost   inde- 

long period. 

Fuel     cost    per     ton 

pendent  of  the  output. 

melted  is  almost  in- 

dependent of  the  out- 

put, when  using  coke. 

Size  of  unit 

Furnaces    can    be    in- 

Minimum capacity  limited 

The   smallest  satisfactory 

and     out- 

stalled   for  any    de- 

to   £    ton,     being    the 

unit  has   a   capacity  of 

put      pos- 

sired output,  however 

smallest   size    commer- 

* ton,  and   cannot  pro- 

sible. 

small. 

cially  practicable.     Pos- 
sible daily  output  much 

duce  such  a  large  daily 
output  as  a  correspond- 

greater   than    with    an 

ing  converter  plant. 

electric       furnace       of 

similar  capacity. 

Suitable 

Generally     very    light 

Suitable  for  medium  light 

Suitable  for  medium  light 

types       of 

castings      only      are 

and  light  cast  ings.  Very 

and  light  castings  of  any 

castings 

made.      Possible    to 

thin  castings  can  be  run, 

description. 

and    tem- 

pour   thin     castings 

but     not    without    risk 

perature 

since   no  appreciable 

of     short-run     wasters. 

limitation. 

heat  loss  on  transfer 

Temperature  of  the  steel 

to  mould. 

depends  upon  the  vari- 

able conlition  of  blow- 

ing, governed  largely  by 

the     condition    of     the 

lining. 

202  THE    ELECTRO-METALLURGY   OF    STEEL 

TABLE  I.— Continued. 


Crucible. 

Side-blown  Converter. 

Electric  Furnace. 

Chemical 

Analysis  rather  irregu- 

Chemical       composition 

Analysis     of    consecutive 

composi- 

lar, due  to  varying  ab- 

under good  control,  giv- 

heats    very     consistent 

tion. 

sorption     of     carbon 

ing  a  consistent  carbon 

owing     to     the      exact 

from  the  pot  ;  sulphur 

percentage.         Sulphur 

chemical  control  during 

and  phosphorus    de- 

and phosphorus  in  the 

refining    and     finishing 

pendent  upon  analy- 

charge increase  slightly 

operations.     Kemoval  of 

sis  of  scrap. 

during  the  blow.     Loss 

P  and  S  possible  by  basic 

of  Mn  and  Si  to  be  al- 

process.     Exact    calcu- 

lowed for  on  addition  of 

lated   additions    of   Mn 

alloys. 

can    be    made    without 

allowing    for    any    oxi- 

dation loss. 

General  pro- 

Crucible steel  castings 

Liable  to  slag   inclusions 

Case-hardens  very  well  up 

perties    of 

do    not    case-harden 

and     dissolved     oxides. 

to  -2  per  cent.  C.     Acid 

castings 

well  if  the  carbon  ex- 

Minute  slag  inclusions 

steel  occasionally  shows 

and  nature 

ceeds    -15    per   cent. 

frequently     cause     test 

slag    inclusions.      Phy- 

of defects. 

Have  a  tendency  to 

pieces  to  fail  either  un- 

sical tests  are  most  con- 

tear or  pull  if  the  steel 

der  the  bend  or  tensile 

sistent     and     are     not 

was   highly   oxidised 

test,     which     increases 

influenced     by     factors 

just    before    pouring. 

the  number  of  wasters. 

other      than      ordinary 

Physical     tests     will 

This   is,   probably,   the 

chemical       composition 

only  be  consistent  in 

most    serious    difficulty 

and      heat      treatment. 

so  far  as  the  chemical 

with     converter     steel. 

The  physical  tests  show 

analyses  are  regular. 

The   physical   tests  are 

a  rather  higher  ductility 

not     so     consistent     as 

than  converter  or   cruc- 

might be  expected  from 

ible     steel     having    the 

the  chemical  uniformity 

same  resistance  to  ten- 

of consecutive  casts  for 

sile     stress.     Owing    to 

the  reasons  above  stated. 

the     unlimited    casting 

Castings    are    liable  to 

temperature  possible  the 

tear      on       contraction 

risk   of    short-run   cast- 

owing to  the  tenderness 

ings    is    reduced    to    a 

due   to   oxides  ;    this  is 

minimum. 

also  augmented  by  high 

sulphur. 

down  for  the  production  of  castings  in  England  was  erected  by 
the  Braintree  Castings  Co.,  Ltd.,  in  1911.  No  considerable 
progress  was  made  anywhere  in  the  development  of  the  electric 
furnaces  for  foundry  purposes  until  1914,  when,  owing  to  the 
rising  price  of  hematite  pig-iron  and  the  availability  of  an 
increasing  supply  of  cheap  steel  scrap,  its  special  merits 
were  more  generally  utilised  for  meeting  the  growing  de- 
mand for  light  intricate  castings.  The  early  electric  steel 
castings  were  far  superior  to  the  best  malleable  iron  castings 
then  being  used  in  the  motor  engineering  trade ;  they  were  of 


APPLICATION  OF  ELECTBIC  FURNACE  TO  FOUNDRY  PRACTICE      203 

uniform  quality  throughout,  could  be  case-hardened,  and  did 
not  suffer  from  the  disadvantage  of  warping,  as  so  commonly 
happens  during  the  malleablising  process.  Drop  forgings  made 
in  complex  dies  were  at  that  time  in  their  infancy,  but  are  now 
used  extensively  in  the  place  of  small  castings,  when  the  shape 
of  the  article  permits.  Owing,  however,  to  this  latter  limitation, 
the  field  for  drop  forgings  must  always  be  restricted. 

The  marked  development  of  the  electric  furnace  for  foundry 
purposes  is  shown  by  the  following  figures,  which  give  the  out- 
put of  castings  during  the  last  few  years  in  Great  Britain  and 
America.  Previous  to  1915  the  returns  for  castings  and  ingots 
produced  in  Great  Britain  were  not  segregrated : — 

1912.  1913.       1914.  1915.  1916.  1917.  1918.         1919. 
Great 

Britain  —  2,000  9,288  15,600  44,901  30,000 

America       4,162  9,207      8,551  23,064  42,870  64,911  58,000  42,000 

The  rapidly  increasing  use  of  the  electric  furnace  in  foundries 
is  not  due  to  any  great  economy  in  the  production  of  the  steel, 
but  rather  to  a  demand  for  a  higher  grade  material,  and  to  the 
considerably  reduced  loss  occasioned  by  short-run,  defective,  and 
rejected  castings.  The  latter  include  castings  returned  from 
machine  shops  and  those  rejected  on  failure  to  comply  with  the 
specified  physical  tests. 

Arrangement  of  Plant. — The  furnace  installation  and  all 
ladle  accessory  plant  are  best  situated  at  one  end  of  a  foundry ; 
all  supplies  may  then  be  handled  and  the  furnace  manipulation 
conducted  so  as  to  cause  the  least  interference  with  work  on 
the  foundry  floor.  The  arrangement  is  also  preferable  from  the 
furnacemen's  point  of  view,  as  the  working  space  around  the 
furnace  is  less  likely  to  be  used  as  a  dumping  ground  for  boxes, 
sand,  and  other  foundry  material.  In  many  shops  there  is  one 
heavy  crane  for  both  furnace  and  casting  use,  and  lighter  ones 
for  the  manipulation  of  boxes  on  the  foundry  floor ;  by  the 
above  arrangement  of  plant,  the  cranes  are  always  situated  in 
a  position  where  they  are  mostly  used,  and  are  therefore  less 
likely  to  interfere  with  one  another  in  their  operation.  A 
battery  of  three  2-ton  single-phase  furnaces  installed  at  the 
works  of  Messrs.  Thwaites  Bros.,  Bradford,  is  shown  in  Fig.  98. 


204  THE   ELECTRO-METALLUEGY   OF    STEEL 

The  furnace  plant  is  erected  at  one  end  of  a  long  foundry  bay  and 
constitutes  an  entirely  self-contained  steel-making  installation. 

Satisfactory  means  of  heating  the  ladle  are  always  provided, 
and,  in  cases  where  hand  shanks  are  used  for  the  transfer  of 
steel  from  the  ladle  to  the  moulds,  special  apparatus  is  installed 
for  their  rapid  and  convenient  heating,  so  as  to  minimise  the 
production  of  "skulls".  Gas  burners  are  sometimes  used  for 
this  purpose,  but  a  simpler  method  consists  of  inverting  each 
shank  over  a  separate  small  coke  or  charcoal  fire,  built  in  an 
unlined  steel  fire-box  provided  with  a  perforated  removable 
bottom  and  adjustable  air  blast.  With  this  appliance  shanks 
can  be  raised  to  correct  temperature  in  half  an  hour.  The  floor 
plates,  which  cover  the  working  floor  space  around  the  furnace, 
are  sometimes  built  a  few  inches  .above  the  shop  floor  level ; 
this  more  readily  enables  a  general  condition  of  cleanliness  to  be 
maintained  round  the  furnace  and  slag  pit. 

For  the  purpose  of  controlling  the  melting  loss  and  general 
efficiency  of  the  furnace,  it  is  necessary  to  know  the  actual 
weight  of  steel  poured  into  the  ladle ;  this  is  a  difficult  figure  to 
arrive  at  accurately  from  the  weight  of  finished  castings,  so  that 
the  use  of  a  dial  crane  hook  weighing  machine  is  always  to  be 
recommended  in  foundry  practice. 

Choice  of  Furnace  Capacity. — At  the  present  time  the  electric 
furnace  is  generally  used  for  the  production  of  light  castings  of 
about  3  cwts.  or  less,  and  has  not  yet  competed  in  the  heavier 
trade  of  open-hearth  castings  in  Great  Britain.  The  number  of 
moulds  laid  down  for  every  ton  of  steel  cast  will  usually  be  con- 
siderable, thirty  to  forty  being  quite  a  common  figure.  Owing 
also  to  the  light  character  of  the  castings,  the  moulds  will  be 
proportionately  large,  so  that  the  floor  space  and  the  number  of 
moulds  required  for  a  ton  of  steel  castings  will  be  greater  than 
in  most  converter  and  open-hearth  plants.  When  a  large 
number  of  moulds  are  laid  down  for  one  cast,  the  majority  will 
be  set  and  remain  on  the  floor  long  before  they  are  actually 
filled,  so  that  the  boxes  are  not  in  constant  use.  It  is  obvious 
that,  by  laying  down  and  casting  a  fewer  number  of  moulds 
which  could  be  promptly  knocked  out  and  returned  to  the 
moulder  for  further  use,  an  economy  of  floor  space  and  moulding 
box  plant  would  naturally  result.  Therefore,  for  a  given  daily 


APPLICATION  OF  ELECTRIC  FURNACE  TO  FOUNDRY  PRACTICE     205 


output  it  is  better  to  install  furnaces  of  small  capacity,  which 
may  be  worked  so  as  to  be  ready  for  casting  at  regular  intervals, 
and  so  collectively  provide  a  frequent  supply  of  steel  to  the 
foundry  in  small  quantities.  The  cost  of  a  steel-making  plant 
consisting  of  several  small  units  will  naturally  be  higher  than 
one  large  unit  having  the  same  daily  output,  but  the  numerous 
advantages  otherwise  gained  will  more  than  compensate  for  the 
increased  establishment  and  labour  charges.  It  may  well  be 
expected  that  the  electric  furnace,  if  operated  in  conjunction 
with  a  basic  open-hearth  or  converter,  will  find  an  extended 
field  of  application  in  the  production  of  heavy  castings  ;  in  this 
event,  furnaces  of  large  capacity  would  be  installed  to  meet  the 
demands  of  the  heavy  engineering  trade. 

Specification  and  Mechanical  Tests. — Castings  are  generally 
ordered  to  fulfil  a  physical  test  specification,  no  restrictions  being 
placed  on  analysis  other  than  for  phosphorus  and  sulphur ;  this 
rule  allows  the  steel-maker  far  more  scope  in  the  choice  of 
analysis,  which  can  be  varied  according  to  the  heat  treatment 
to  which  the  castings  are  submitted. 

The  standard  specifications  for  steel  castings  adopted  by  the 
American  Society  for  Testing  Materials  in  1912  are  given  below : — 


Hard.' 

Medium. 

Soft. 

Tensile  stress  per  sq.  inch 

36-0  tons 

31-0  tons 

27-0  tons 

Yield  point       

16-0     „ 

14-0     „ 

13-0     „ 

Elongation  on  2  inches     . 

15  per  cent. 

18  per  cent. 

22  per  cent. 

Reduction  of  area     .... 

20 

25       „ 

30 

Cold  bend  1  inch  x  £  inch 

— 

90° 

120° 

All  castings  to  be  annealed  and  slowly  cooled. 

Some  American  Marine  specifications  are  more  exacting 
than  the  above,  as  shown  by  the  following  example:— 

Tensile  stress,  36*0  tons  ;  Yield,  point,  18*0  tons;  Elongation 
on  2  inches,  17  per  cent. ;  Reduction  of  area,  25  per  cent. ;  Bend, 
90°  (1  inch  x  ^  inch).  The  above  specifications  are  intended 
for  steel  castings  with  phosphorus  and  sulphur  below  '05  per 
cent,  and  are  easily  met  by  electric  steel  made  either  from  cold 
scrap  charges  or  by  the  duplex  process.  In  America  all  Govern- 
ment castings  are  ordered  in  the  annealed  condition. 


206  THE    ELECTKIOMETALLUBGY   OF   STEEL 

The  specifications  adopted  by  the  British  Engineering 
Standards  Committee  for  marine  castings  are  very  similar  to  the 
above,  but  do  not  generally  apply  to  the  light  intricate  variety 
of  automobile  and  other  such  castings.  When  specifications  are 
given,  it  will  be  seen  from  the  figures  given  below  how  easily  they 
can  be  met  and,  moreover,  outclassed  to  an  extent  which  places 
electric  steel  castings  in  a  special  category.  The  following  are 
typical  tests  of  electric  steel  castings  made  by  the  basic  process 
from  cold  scrap  charges  : — 

Max.  Stress,        Yield  Point,        Per  Cent.  Per  Cent, 

tons  per  tons  per  Elongation  Reduction 

sq.  inch.  sq.  inch.  on  2  inches.  of  Area. 

I.  Analysis,  C  -15 ;  Mn  -1 ;  Si  -21. 
(a)  As  cast  34  3  23-0  26 

31-1  21-8  28-5 

II.  Analysis,  C  -2--25;  Mm  -5--5S;  Si  -3--3S. 

(a)  As  cast  30-0  16 

(6)  Annealed  at  900°  C.  to  950°  C.  and  slowly  cooled. 

31-71  22-14  34  52-7 

35-0  23-18  23  34-6 

(c)  Same  treatment  as  (b)  but  from  a  large  casting. 

31-07  17-73  33-25  52-2 

27-01  18-75  38-5  57-8 

(d)  Annealed,  water  quenched  from  750°  C.  and  tempered  at  550°  C. 

39-05  27-93  22-0  38-3 

III.  Analysis,  C  -4  per  cent. ;  Mn  -5  per  cent. ;  Si  -25  to  '35. 
(a)  Annealed  and  cooled  in  air. 

40-09  27-01  17-25  21-1 

42-6  24-4  20-0 

Acid  steel  made  from  scrap  low  in  phosphorus  and  sulphur 
will  give  somewhat  similar  physical  test  results.  At  one  British 
foundry,  equipped  with  several  acid  lined  furnaces,  the  steel  is 
practically  standardised  and  made  to  the  following  analysis: — 

C          .         .         .         .  '25  -  *3    per  cent. 

Mn -8 

Si  -3 

P  and  S  under  '04        ,,          each. 

The  castings,  which  are  very  light  and  intricate,  are  all 
annealed  at  900°  C.  for  1-|  hours  and  air  cooled ;  test  pieces 
submitted  to  this  treatment  regularly  give  the  following  physical 
test  results : — 


APPLICATION  OF  ELECTRIC  FURNACE  TO  FOUNDRY  PRACTICE     207 

Max.  stress     ......     30/35  tons. 

Yield  point      ....  .     18/21      „ 

Per  cent,  elongation        ....     17/20      „ 

,,         reduction  area          .         .         .30  per  cent. 

Annealing:. — The  simplest  process  of  annealing  castings  con- 
sists of: — 

1 .  Slow  reheating  to  a  temperature  above  the  highest  arrest 
or  Ac  point. 

2.  Soaking  at  that  temperature,  so  as  to  break  down  the 
coarse  crystalline  structure. 

3.  Slow  cooling  through  the  recalescence   points  to  about 
200°  C.  to  300°  C.,  after  which  rapid  cooling  in  air  is  permissible. 
The  above  treatment  is  sometimes  varied  by  rapidly  cooling  the 
casting   from  the  soaking   temperature  to  that  of  the   lowest 
recalescence  point,  which  means  a  drop  of  about  150°  C.  for 
mild  steel,  and  then  slowly  cooling  from  that  temperature ;  this 
treatment  gives  a  finer  structure  than  if  the  steel  were  allowed 
to  cool  slowly  from  the  soaking  temperature  down  to  its  low- 
est recalescence  point. 

If  an  unannealed  casting  is  examined  microscopically,  it  will 
be  found  that  the  structure  is  either  coarsely  granular  or  else 
exhibits  a  large  irregular,  triangular  shaped  pattern.  Both 
these  structures  are  due  to  the  prolonged  high  temperature  to 
which  the  steel  is  exposed  after  solidification,  and  differ  only 
by  reason  of  slow  or  rapid  cooling  respectively  from  the  tem- 
perature of  the  highest  to  that  of  the  lowest  recalescence  point. 
This  coarse  structure  is  a  source  of  weakness,  as  the  ferrite  areas 
are  more  easily  torn  apart  than  when  irregular  in  form.,  more 
finely  disseminated,  and  more  closely  interlinked.  By  soaking 
such  cast  steel  at  a  temperature  just  above  the  highest  arrest 
or  Ac  point  these  coarse  crystalline  grains  are  broken  down, 
and  complete  diffusion  of  the  carbon  results.  Slow  cooling  from 
this  temperature  allows  the  excess  of  ferrite  to  fall  out  of  solution 
again,  with  total  elimination  of  the  original  structure. 

The  microphotographs  of  similar  magnification  shown  in 
Figs.  99  and  100  illustrate  the  marked  change  brought  about  in 
the  crystalline  structure  of  a  mild  steel  casting  by  an  annealing 


208  THE   ELECTRIC-METALLURGY  OF   STEEL 

treatment.  The  steel  contained  *2  per  cent.  C ;  '22  per  cent. 
Si ;  '53  per  cent.  Mn,  and  it  will  be  seen  how  the  coarse 
crystalline  structure  of  the  steel  as  cast  has  been  entirely 
changed  to  one  consisting  of  finely  disseminated  and  irregular 
shaped  patches  of  ferrite  and  pearlite.  The  marked  improve- 
ment in  the  ductile  properties  of  this  particular  cast  of  steel 
resulting  from  this  change  of  structure  is  shown  by  the  follow- 
ing test  results:  — 

As  Cast.  After  Amiealing . 

Ultimate  stress  per  sq.  in.  3O46  tons.  30*9  tons. 

Yield  point         .         .         .  18'8     „ 

Elongation  on  2  ins.  .  18*0  per  cent.  34'0  per  cent. 

Per  cent,  reduction  of  area  20'0        ,,  40'0        ,, 

It  is  obvious  from  these  figures  that  a  steel  casting  in  an 
annealed  condition  will  be  far  more  resistive  to  sudden  fracture 
by  shock  than  when  in  the  "  green  "  state. 

The  grain  size  of  annealed  steel  will  depend  upon  (1)  the 
soaking  temperature  to  which  the  steel  is  raised ;  (2)  the  rate 
of  cooling  from  the  soaking  temperature  to  that  of  the  lowest 
recalescence  point ;  (3)  the  rate  of  cooling  from  the  lowest  re- 
calescence  point  to  atmospheric  temperature. 

The  temperature  of  annealing  rises  from  800°  C.  to  850°  C. 
for  medium  carbon  steels  up  to  900°  C.  to  950°  C.  for  mild  steel, 
while  higher  soaking  temperatures  tend  to  produce  a  coarser 
grain.  Importance  is  not  usually  attached  to  the  effect  of  rapid 
cooling  from  the  soaking  temperature  to  that  of  the  lowest 
recalescence  point,  but  in  some  cases  the  practice  is  followed  of 
opening  the  annealing  stove  doors  to  cause  rapid  cooling,  and 
then  closing  them  up  again  until  the  castings  are  ready  for  re- 
moval. It  is  generally  acknowledged  that  the  finer  the  grain 
of  an  annealed  sample  of  steel  the  better  will  be  its  physical 
properties,  and  to  secure  this  in  practice  the  castings  are  often 
withdrawn  from  the  annealing  stove  and  cooled  in  air.  Excellent 
tests  may  be  obtained  in  this  way,  but  unfortunately  there  are 
risks  of  setting  up  internal  stresses  in  castings  which  vary  in 
thickness,  owing  to  the  rapid  and  irregular  rate  of  cooling.  In 
America  this  point  is  considered  of  sufficient  importance  to 
warrant  a  general  stipulation  that  all  annealed  castings  shall  be 


FIG.  99. 


FIG.  100. 


\To  face  p.  208. 


APPLICATION  OF  ELECTBIC  FURNACE  TO  FOUNDRY  PRACTICE      209 

slowly  cooled.  Castings  cooled  slowly  will  give  a  poorer  test 
generally,  but  will  be  more  reliable  in  service. 

The  process  of  annealing  also  removes  the  unequal  contrac- 
tion stresses  set  up  in  a  casting  during  its  initial  cooling ;  this 
applies  particularly  to  castings  that  vary  considerably  in  thick- 
ness, and  which  may  show  "  pulls  "  due  to  this  cause.  There 
is  considerable  diversity  of  opinion  as  regards  the  advisability 
of  annealing  dead  soft  steel  castings  containing  less  than  '^5 
per  cent,  carbon.  It  cannot  be  denied  that  annealing  will  im- 
prove the  tensile,  yield  point,  and  elongation  figures  of  all 
carbon  steel  castings,  by  converting  the  original  coarse  struc- 
ture into  one  of  finer  grain  which  produces  a  fibrous  fracture. 
However,  rough  test  pieces  cast  \  inch  thick  from  such  low 
carbon  steel  should  bend  double  when  cold,  so  that,  so  far  as 
ductility  and  resistance  to  shock  are  concerned,  castings  made 
in  this  low  carbon  material  will,  in  the  "  green  "  state,  fulfil  the 
usual  requirements.  Therefore,  unless  annealing  is  actually 
specified  there  seems  little  reason  to  incur  the  added  expense  of 
improving  the  quality  beyond  what  is  required  by  the  user  for 
the  particular  purpose  intended.  The  question  of  annealing 
dead  soft  castings  must  then  be  left  to  individual  manufacturers 
to  base  their  decision  according  to  the  demand.  Unannealed 
castings,  as  has  been  stated,  will  always  be  unequally  stressed, 
and  their  quality  in  this  respect  will  be  improved  by  heating  to 
a  few  hundred  degrees  centigrade. 

Defects  of  Steel  Castings. — The  production  of  sound  steel 
castings  depends  far  more  upon  the  art  of  moulding  than  of 
steel-making,  and  for  the  purpose  of  remedying  defects  a  careful 
distinction  must  be  drawn  between  their  possible  causes.  De- 
fects in  castings  may  be  sometimes  due  to  an  improper  condition 
of  the  steel  when  cast,  but  are  more  generally  caused  by 
unsuitable  methods  of  casting,  more  especially  in  respect  of  the 
nature  and  construction  of  the  sand  mould.  Careful  examina- 
tion of  a  defective  casting  will  usually  provide  definite  evidence 
as  to  the  cause  of  the  defect,  which  in  certain  cases  can  be 
remedied  by  suitable  modification  of  the  composition  and  physical 
condition  of  the  steel  when  cast. 

Blowholes. — The  term  "  bio  whole"  is  generally  applied  to 
cavities  formed  by  the  generation  of  gas,  which  accompanies 

14 


210  THE   ELECTRO-METALLURGY   OF    STEEL 

the  chemical  reaction  between  the  carbon  constituent  of  the 
steel  and  dissolved  iron  oxide  that  takes  place  on  solidification. 
In  the  case  of  ingots  such  unsoundness  is  generally  due  to  the  im- 
perfect removal  of  the  dissolved  oxides  from  the  steel  before  cast- 
ing. This  does  not  always  apply  in  the  case  of  castings,  since  it 
is  quite  possible  for  sufficient  oxide  to  be  formed  and  absorbed  by 
the  liquid  steel  in  the  moulds,  owing  to  generation  of  steam  from 
a  damp  sand  skin.  The  blowholes  formed  by  imperfectly  de- 
oxidised steel  will  be  found  distributed  irregularly  throughout 
the  casting  from  its  skin  inwards,  and  will  usually  be  lined  with 
a  thin  colour  film  of  iron  oxide.  An  unsound  casting  will  have 
unusually  sharp  edges,  owing  to  the  expansion  of  the  outside 
skin,  which  is  caused  by  the  internal  pressure  of  the  gases 
generated  as  solidification  proceeds.  Steel  that  may  produce  a 
perfectly  sound  casting  in  dry  sand  will  sometimes  blow  when 
cast  into  imperfectly  skin-dried  "green  sand"  moulds;  this  is 
due  to  the  steam  being  generated  from  the  moisture  present, 
which  causes  local  oxidation  unless  there  is  sufficient  silicon 
and  aluminium  in  the  steel  to  neutralise  its  oxidising 
influence.  Blowholes,  which  are  thus  due  to  the  action  of 
water  vapour,  are  generally  confined  to  the  region  of  the  skin, 
and  do  not  persist  to  the  centre  of  the  casting  in  other  than 
exceptionally  bad  cases.  If  the  trouble  cannot  be  remedied  by 
ensuring  that  every  mould  is  properly  dried,  it  may  be  greatly 
mitigated  by  a  more  liberal  addition  of  aluminium  to  the  steel. 
Silicon  will  also  assist  in  the  same  way  as  aluminium,  but  its 
action  is  not  so  certain  and  rapid.  For  the  above  reason  alone 
it  is  quite. a  common  practice  to  add  one  pound  of  aluminium  to 
a  ton  of  steel  in  the  ladle ;  this  applies  equally  to  electric, 
crucible  and  converter  steels. 

Short-run  Castings. — When  steel  is  poured  into  a  sand 
mould,  it  may  happen  that,  owing  to  insufficient  temperature 
or  to  the  thinness  of  the  pattern,  it  fails  to  fill  all  parts  of  the 
mould  entirely  and  produces  a  "  short-run "  casting.  For 
certain  engineering  purposes  lightness  may  be  of  prime  im- 
portance, so  that  it  is  only  by  increasing  the  casting  tempera- 
ture, and  not  by  slightly  thickening  the  pattern,  that  this 
difficulty  can  be  met.  To  obtain  a  very  high  casting  tempera- 
ture may  entail  prolonged  heating  with  increased  power  con- 


APPLICATION  OF  ELECTEIC  FUENACE  TO  FOUNDEY  PEACTICE      211 

sumption,  so  that  it  is  generally  more  economical  to  reserve  as 
many  as  possible  of  the  lightest  castings  for  one  heat,  which 
may  be  cast  specially  hot  for  the  purpose. 

Gas  Cavities. — Insufficient  venting  or  porosity  of  the  sand 
may  prevent  the  rapid  and  complete  displacement  of  air  from 
the  moulds  while  filling,  in  which  case  bubbles  of  gas  may  become 
entrapped,  and  remain  under  the  skin  on  the  "  cope  "  side  of 
the  casting.  Large  and  isolated  cavities  resulting  from  this 
cause  cannot  be  confused  with  blowholes  formed  by  oxidation 
of  the  steel  before  solidification. 

Brittleness. — This  defect  is  entirely  independent  of  the 
method  of  moulding  and  pouring,  and  is  solely  due  to  the  con- 
dition of  the  steel  when  cast.  Brittleness  of  a  cold  casting  may 
be  due  to  a  moderately  high  carbon  content,  in  which  case 
annealing  will  often  suffice  to  remedy  entirely  a  defect  which  is 
then  only  apparent  in  the  unannealed  condition.  Occasionally 
brittleness  will  be  exhibited  in  a  mild  steel  casting,  and  is  then 
characterised  by  a  bright  coarsely  crystalline  fracture.  This 
condition  is  usually  due  to  a  high  silicon  content,  which  occurs 
more  especially  in  acid  electric  steel  owing  to  the  reduction  of 
silicon  from  the  slag  under  highly  reducing  conditions.  Anneal- 
ing will  generally  suffice  to  remedy  this  fault,  which,  however, 
can  be  prevented  by  careful  control  of  the  silicon  alloy  additions 
and  the  condition  of  the  slag  during  the  finishing  operations. 

Slag  Inclusions. —  Slag  inclusions  are  sometimes  present 
in  both  acid  and  basic  steel  as  very  minute  particles,  which 
under  the  microscope  are  found  lying  along  the  boundaries  of 
the  crystal  grains.  The  slag  particles  usually  appear  strung 
together  like  a  chain  of  beads,  three  such  chains  usually  meet- 
ing in  a  common  point  of  intersection.  The  grains,  whose 
boundary  lines  they  mark,  are  those  which  result  from  the 
primary  formation  of  pure  equiaxed  iron  crystals  during  the 
process  of  solidification,  and  which  continue  to  grow  outwardly 
in  all  directions  until  they  meet  one  another.  During  the 
process  of  cooling,  subsequent  to  solidification  and  diffusion  of  the 
carbon  into  the  iron  crystals,  these  slag  particles  exert  a  selective 
attraction  for  the  ferrite  constituent,  so  that  in  low  carbon 
steels,  they  are  found  embedded  in  an  area  of  pure  iron  from 
which  the  carbon  bearing  constituent  "pearlite"  is  totally 


212  THE    ELECTEO-METALLUEGY   OF    STEEL 

absent ;  Fig.  101  shows  their  appearance  at  a  magnification  of 
100  diameters.  These  chains  of  slag  inclusions  cause  planes  of 
weakness,  since  the  metal  that  divides  their  particles  and  inter- 
links the  neighbouring  grains  consists  of  weak  ferrite.  Steel 
containing  these  inclusions  frequently  fails  under  mechanical 
test  in  spite  of  satisfactory  chemical  analysis  and  heat  treat- 
ment. 

These  minute  slag  inclusions  owe  their  origin  to  silicates, 
which  exist  in  the  liquid  steel  either  in  a  state  of  solution 
or  more  probably  in  an  emulsified  form  at  the  time  of  casting. 
During  the  process  of  solidification  the  silicates  are  expelled 
from  the  liquid  steel,  and  coalesce  to  form  minute  chains  of 
segregates.  Highly  siliceous  compounds  have  considerable 
power  of  coalescence,  and  were  it  not  for  this  property,  it  is 
doubtful  whether  these  slag  inclusions  would  ever  become 
microscopically  visible  as  such  definitely  arranged  segregates. 

It  is  also  possible  that  part  of  the  silicates  are  not  formed  until 
crystallisation  of  the  ferrite  begins,  which  then  allows  a  reaction 
between  the  silicon  and  any  dissolved  metallic  oxides  to  proceed. 
In  this  way  the  formation  and  segregation  of  the  silicates  would 
be  simultaneous.  Kepeated  annealing  is  sometimes  sufficient 
to  break  down  these  ferrite  areas,  but  only  after  actual  dis- 
placement of  the  slag  particles. 

Shrinkage  Cavities  or  "  Draws  ". — Cavities  due  to  shrinkage 
of  the  steel  during  solidification  in  the  moulds  are  analogous  to 
"  pipes  "  in  ingots,  and  are  generally  governed  by  the  same  laws 
of  formation.  The  prevention  of  shrinkage  cavities  is  a  question 
of  moulding,  but  may  be  assisted  to  a  slight  extent  by  reducing 
the  casting  temperature  and  the  percentages  of  manganese  and 
silicon  to  a  minimum,  consistent  with  other  necessary  factors. 

Tears,  Cracks,  or  Pulls. — This  particular  form  of  defect  is 
due  to  failure  of  the  steel  to  resist  rupture  under  the  tensile 
stresses  developed  during  either  equal  or  unequal  contraction 
on  cooling.  Here  again,  the  remedy  usually  lies  in  the  method 
of  moulding  or  alteration  of  the  pattern,  when  possible. 
Naturally,  steel  that  is  tough  at  a  high  temperature  will  best 
resist  fracture  under  tensile  stress,  and  will  then  either  yield 
uniformly  itself,  or  compress  portions  of  the  sand  mould  and 
cores  so  as  to  accommodate  itself  freely  to  reduced  dimensions 


FIG.  101. 


[To  face  p.  212. 


APPLICATION  OF  ELECTRIC  FURNACE  TO  FOUNDRY  PRACTICE      213 

without  distortion.  Steel  that  is  most  resistant  to  this  type  of 
defect  will  have  a  high  degree  of  chemical  purity,  being  as  free 
as  possible  from  dissolved  oxides  and  sulphur,  both  of  which 
cause  "red-shortness"  or  brittleness  at  high  temperatures. 
It  is,  therefore,  also  in  the  hands  of  the  steel-maker  to  obviate 
these  defects,  which  are  least  pronounced  in  basic  electric  steel. 


CHAPTEK  XI. 

CHARACTERISTIC  FEATURES  AND  PRINCIPLES  OF  FURNACE  DESIGN. 

THE  design  of  electric  arc  furnaces  for  steel-making  involves  a 
careful  study  of  the  special  metallurgical  conditions  required, 
and  of  the  electrical  and  mechanical  means  by  which  these  con- 
ditions may  best  be  fulfilled  with  simplicity,  economy,  and 
regularity  of  operation. 

An  electric  furnace  for  steel-making  is  above  all  a  metal- 
lurgical appliance,  and  as  such  must  be  designed  as  far  as 
possible  in  accordance  with  certain  conditions  imposed  by  the 
particular  process  adopted.  It  is  obviously  impossible  to 
embody  in  any  furnace  every  feature  that  is  desirable  from  a 
metallurgical  standpoint,  as  this  could  only  be  done  by  sacrific- 
ing certain  fundamental  principles  of  the  mechanical  and 
electrical  design.  The  result  then  must  invariably  be  a  com- 
promise, in  which  the  mechanical  and  electrical  features  are  to 
a  certain  extent  subordinated  to  and  ruled  by  the  metallurgical 
conditions  imposed.  For  this  reason  it  will  be  useful  to  outline 
the  chemical,  physical,  electrical  and  mechanical  conditions, 
which  are  either  purposely  or  unavoidably  produced  during  the 
basic  or  acid  process  of  steel-melting  and  refining,  and  to  point 
out  to  what  extent  these  conditions  influence  the  general  con- 
struction and  lining  of  a  furnace  installation. 

Chemical  Conditions. — (a)  The  various  chemical  reactions 
between  basic  slag  and  liquid  steel  proceed  in  most  furnaces 
under  the  influence  of  intense  local  heat,  which,  especially  under 
reducing  conditions,  causes  slight  volatilisation  of  the  slag  con- 
stituents ;  the  basic  fumes  so  formed  have  a  marked  tendency 
to  flux  the  acid  portion  of  the  furnace  lining,  namely,  the  silica 
roof  and  walls. 

(6)  A  direct  arc  striking  downwards  on  to  a  slag  blanket  at 
an  inclined  angle  frequently  causes  rotation  about  the  axis  of 

(214) 


FEATURES  AND   PRINCIPLES  OF   FURNACE   DESIGN          215 

the  electrode,  and  in  small  furnaces,  where  the  walls  are  usually 
close  to  the  arc,  this  circulation  may  cause  considerable  erosion 
of  the  hearth  at  the  slag  line.  This  action  is,  of  course,  greatly 
influenced  by  the  temperature  and  the  corrosive  power  of  the 
slag. 

(c)  Certain  operations,  notably  that  of  carburising,  must  be 
performed  when  the  bath  of  metal  is  free  from  any  slag  cover- 
ing, and  it  is  also  necessary  in  many  cases  to  remove  one  slag 
prior  to  the   formation  of   another.     The  removal  of   slag  by 
skimming  would  be  an  almost  impossible  operation  to  perform 
in  a  fixed  furnace,  more  especially  as  the  level  of  the  slag  line  is 
not  always  the  same  in  successive  heats. 

(d)  Chemical  erosion   of  the  bottom   is   sometimes  severe, 
owing  to  the  close  proximity  of  the  arc  zones  during  the  melt- 
ing down  stage  and  the  local  generation  of  heat  in  the  hearth 
of  conductive  bottom  furnaces.    The  bottom  has,  then,  a  tendency 
to  become  deeper,  so  that  it  would  be  impossible  always  to  drain 
a  furnace  provided  with  a  fixed  taphole ;  this  would  occasion 
serious  difficulties  owing  to  the  possible  contaminating  influence 
of  a  residual  quantity  of  an  alloy  steel  on  a  subsequent  charge, 
and  also  to  the  impossibility  of  effecting  any  repair  to  the  bottom, 
so  long  as  any  steel  covered  the  worn  or  damaged  portion. 

(e)  Burnt-lime,  which  is  generally  used  as  a  flux  in  the  basic 
process,  always  contains  a  quantity  of  slaked  powder.  On 
charging  this  flux  into  a  hot  furnace,  the  light  dust  rises  with 
the  natural  upward  current  of  air,  and  so  comes  into  contact 
with  the  roof,  which,  being  almost  invariably  made  of  silica  brick, 
is  liable  to  be  fluxed.  To  avoid  this  action,  limestone  has  fre- 
quently been  substituted  for  lime,  although  certain  disadvantages 
that  are  then  introduced  hardly  justify  its  use.  Experience  also 
shows  that  the  erosion  of  the  silica  brick  is  worst  at  that  point 
where  these  dust-laden  gases  escape  from  the  furnace  around 
the  electrodes  ;  here  the  annular  opening  is  constricted  and  the 
increased  velocity  of  the  gas  escaping  under  pressure  accentuates 
the  fluxing  action.  Any  means  by  which  these  gases  could  be 
prevented  from  so  escaping  would  prolong  the  life  of  that  part 
of  the  lining. 

(/)  The  various  metallurgical  processes  depend  upon   de- 
finite chemical  reactions,  which   must   be   allowed   to  proceed 


216  THE   ELECTRO-METALLURGY   OF   STEEL 

uninfluenced  by  chemical  conditions  other  than  those  purposely 
introduced.  Mechanical  disintegration  of  a  furnace  hearth  or 
roof  will  invariably  exert  a  contaminating  influence  upon  the 
metal  and  slag,  and  in  certain  cases  will  render  it  impossible  to 
maintain  the  desired  chemical  conditions.  Such  disintegration 
and  failure  of  refractory  materials  may  also  be  caused  by  im- 
proper treatment  during  their  preliminary  heating,  as  has  been 
explained  elsewhere.  Care  must  therefore  be  exercised  in  the 
choice  of  suitable  materials  and  in  the  method  of  employing 
them  for  the  hearth  and  roof  construction. 

The  highly  reducing  properties  of  certain  slags  will  be  vitiated, 
if  not  destroyed,  by  the  oxidising  influence  of  air,  when  allowed 
to  enter  the  furnace  too  freely.  This  demands  attention  to  door 
construction  and  to  the  restriction,  as  far  as  possible,  of  the 
annular  openings  surrounding  the  electrodes,  as  the  free  escape 
of  gases  at  this  point  induces  natural  convection  currents  of  air 
through  the  furnace. 

(</)  The  chemically  corrosive  action  of  acid  and  basic  slags 
on  furnace  banks  is  sometimes  considerable,  more  especially 
when  they  are  highly  charged  with  iron  oxide  derived  from  rusty 
scrap.  Suitable  refractory  material  must  be  chosen  to  withstand 
as  far  as  possible  the  corrosive  action  of  such  slags,  and  the 
natural  slope  of  the  hearth  at  the  slag  line  should  be  such  as  to 
permit  ease  of  fettling  after  every  heat. 

(li)  The  various  reactions  between  carbon  and  oxygen,  which 
proceed  under  the  oxidising  and  reducing  slag  conditions,  are 
accompanied  by  liberation  of  carbon  monoxide.  This  gas  is  at 
times  generated  so  rapidly  that  it  is  forced  to  escape  through 
the  door  crevices  under  slight  pressure.  It  is  also  often  charged 
with  iron  oxide  fume  during  the  melting  down  stage,  and  there- 
fore causes  erosion  of  the  silica  brick  lining  at  the  various  points 
of  escape. 

For  this  reason  the  door  arches  and  roof  openings  tend  to 
suffer  most,  and  it  is  therefore  advisable  to  co'nstruct  the  former 
in  such  a  way  that  they  can  be  renewed  without  damaging  the 
adjoining  walls. 

Physical  Conditions. — (a)  To  satisfy  various  metallurgical 
conditions  it  is  necessary  to  raise  a  bath  of  steel  to  a  definite 
temperature,  the  maximum  being  usually  dependent  upon  the 


FEATURES  AND  PRINCIPLES  OF  FURNACE   DESIGN          217 

casting  temperature  desired.  From  the  enclosed  and  confined 
nature  of  electric  furnaces,  and  under  certain  methods  by  which 
the  arc  heating  is  applied,  the  temperature  of  the  upper  lining 
may  be  considerably  higher  than  the  bath  of  steel  when  ready  for 
casting.  Such  excessive  lining  and  roof  temperatures,  when 
impossible  to  obviate,  are  due  to  electrical  and  mechanical 
conditions  which  produce  unshielded  or  flaming  arcs.  With 
an  ideal  mode  of  electric  arc  heating  the  heat  generated  would 
be  entirely  absorbed  by  the  furnace  charge  alone,  so  that  the 
furnace  walls  and  roof  would  only  receive  heat  by  radiation  from 
the  heated  charge.  In  practice  this  is  impossible  for  arc 
furnaces,  and  in  all  cases  parts  of  the  upper  furnace  lining  are 
more  or  less  exposed  to  the  directly  radiated  heat  of  the  arc 
itself.  Under  the  best  conditions  such  heating  is  only  local,  being 
confined  to  the  lower  portion  of  the  walls  above  the  slag  line, 
and  nearest  the  arc.  Heat  is  also  reflected  upwards  from  glassy, 
thin  slags,  and  may  at  times  be  sufficiently  intense  to  cause 
fusion  of  the  brickwork. 

It  can  be  broadly  stated  that  the  intensity  and  distribution 
of  heat  in  all  arc  furnaces,  other  than  the  largest  sizes,  is  injurious 
to  the  roof  and  wall  lining,  and  for  this  reason  the  refractory 
materials  require  to  be  most  carefully  chosen  and  utilised. 

(6)  Sudden  and  considerable  variations  of  temperature  are 
equally  important  in  their  effect  upon  refractory  materials,  and 
in  this  respect  the  electric  furnace,  when  used  for  melting  cold 
scrap,  is  at  a  disadvantage  in  comparison  with  other  steel-melt- 
ing furnaces.  The  door  area  is  relatively  large  for  the  purpose 
of  charging  scrap,  and  the  lining  is  very  rapidly  chilled  during 
the  interval  between  heats. 

Owing  to  the  serious  influence  of  sudden  temperature  changes, 
the  selection  of  suitable  brands  of  basic,  acid,  or  neutral  bricks 
should  be  guided  by  their  power  to  resist  fracture  under  such 
variable  conditions  of  service.  Bauxite  and  certain  brands  of 
magnesite  brick  are  well  able  to  stand  this  treatment,  whereas 
others  fail. 

(c)  The  distribution  of  heat  radiated  from  the  arcs  will 
depend  upon  their  number  and  position,  and  in  every  case  the 
contour  of  the  lining  should  be  so  designed  that  no  part  of  the 
brickwork  is  unduly  placed  under  the  influence  of  the  intense 


218  THE   ELECTKO-METALLURGY   OF   STEEL 

arc  temperature.  The  shape  of  the  furnace  body  is  therefore 
dependent  upon  the  number  and  disposition  of  the  arcs ;  when 
the  principle  of  direct  arc  heating  is  used,  a  circular  form  is 
favoured  for  either  single,  three,  or  four  arcs,  and  a  rectangular 
shape  for  two  arcs.  In  the  case  of  indirect  arc  heating,  the 
heating  zones  are  located  between  the  extremities  of  two  or 
three  electrodes,  which  in  the  latter  case  converge  upon  a 
common  point ;  the  shape  of  the  body  is  circular  or  rectangular 
according  to  the  particular  arrangement  of  the  electrodes.  The 
uniform  distribution  of  heating  zones  becomes  an  important 
factor  in  the  design  of  large  furnaces,  owing  to  the  desirability 
of  always  melting  under  the  direct  influence  of  the  arc,  rather 
than  by  heat  reflected  from  over-heated  portions  of  the  walls 
and  roof,  and  conducted  through  the  charge ;  at  the  same  time, 
uniformity  of  bath  temperature  is  desirable  even  during  the 
melting  stage,  and  for  these  reasons  furnaces  having  only  two 
arc  zones  are  unsuitable  for  large  capacities.  The  position  of 
the  heating  zones  relative  to  the  furnace  lining  is  also  of  great 
importance,  and  should  really  serve  as  a  starting  point  from 
which  all  other  constructional  details  are  evolved. 

(d)  Heat  loss  by  radiation  from  the  furnace  body  and  elec- 
trodes demands  careful  attention  in  furnace  design. 

Metallurgical  conditions  demand  that  definite  bath  and  slag 
temperatures  shall  be  reached,  and,  so  long  as  such  temperature 
conditions  are  satisfied  without  introducing  other  outside  in- 
fluences, the  several  processes  of  steel-making  are  technically 
possible.  To  be  economically  possible  is  a  different  matter, 
and  the  rate  at  which  the  desired  temperature  is  reached,  and 
not  the  temperature  itself,  then  becomes  of  vital  importance. 
Power  supplied  in  the  form  of  electrical  energy  is  converted  into 
heat  at  restricted  zones  within  the  furnace,  and,  since  electrical 
heating  is  the  most  extravagant  form  used  in  commerce,  it  must 
be  used  with  the  strictest  economy  possible,  and  every  possible 
attention  paid  to  heat  loss.  The  heat  radiated  from  any  hollow 
body  in  which  heat  is  internally  developed,  depends  upon : — 

(i)  The  temperature  difference  between  the  inner  and  outer 
surfaces  of  the  containing  walls. 

(ii)  The  thermal  conductivity  and  thickness  of  the  material 
of  which  the  walls  are  composed. 


FEATUEES  AND  PKINCIPLES  OF  FUENACE  DESIGN    219 

(iii)  The  total  area  of  the  external  radiating  surfaces. 

Of  the  above  conditional  factors  the  first  is  fixed  by  the 
internal  temperature  required  and  that  of  the  surrounding 
atmosphere,  and  is  therefore  outside  the  possibility  of  control ; 
the  total  radiation  loss  as  affected  by  the  other  factors  is,  how- 
ever, capable  of  being  to  a  great  extent  controlled. 

The  rate  at  which  heat  units  pass  at  all  points  from 
inside  the  furnace  to  the  outer  shell  is  dependent  upon  the 
thermal  conductivity  of  the  refractory  lining  and  its  thickness. 
Suitable  refractories  for  lining  purposes  cannot  be  chosen  alone 
from  the  point  of  view  of  low  thermal  conductivity,  so  that  the 
only  means  of  raising  the  total  thermal  resistance  is  by  in- 
creasing the  thickness  of  the  lining,  or  by  inserting  a  heat 
insulating  backing,  consisting  of  infusorial  earth  or  bricks  made 
of  this  substance,  between  the  lining  and  the  furnace  shell.  By 
adopting  such  measures  the  heat  insulation  of  the  furnace  may 
be  improved,  with  a  consequent  lowering  of  the  heat  loss. 
There  is,  unfortunately,  a  limiting  factor  to  this  simple  pro- 
cedure. The  lining  of  arc  furnaces,  other  than  the  largest,  is 
always  more  or  less  locally  exposed  to  intense  heat  radiated 
directly  from  the  arc,  and  for  this  reason  such  parts  of  the 
lining  will  be  rapidly  fused,  unless  the  heat  is  capable  of  being 
withdrawn  fast  enough  to  lower  the  temperature  of  exposed 
surfaces.  Therefore,  careful  heat  insulation  of  small  furnaces 
will  reduce  the  radiation  loss,  but  ^  only  at  the  expense  of  a 
shortened  life  of  the  lining.  Increasing  the  thickness  of  furnace 
walls  increases  at  the  same  time  the  area  of  the  furnace  body 
and  consequently  the  surface  of  heat  radiation ;  this  further 
limits  the  thickness  of  linings  for  furnaces  of  small  capacity 
and  low  power  input. 

From  the  standpoint  then  of  heat  economy  the  following 
points  in  the  design  of  electric  furnaces  are  worthy  of  note : — 

(i)  The  shape  of  the  furnace  body  and  roof  should  ap- 
proximate to  a  sphere,  which,  for  a  given  internal  capacity,  has 
a  smaller  external  surface  than  any  other  geometrical  solid. 
Eound  or  octagonal  furnace  bodies  are  fairly  close  approxima- 
tions, and  are  better  than  a  rectangular  form. 

(ii)  The  thickness  of  the  lining  should  be  chosen  ac- 
cording to  the  distance  between  it  and  the  arc  zones ;  in  large 


220  THE   ELECTEO-METALLUEGY   OF    STEEL 

furnaces  the  silica  lining  is  sufficiently  remote  to  be  uninfluenced 
by  the  intense  radiated  heat,  and  is  only  subjected  to  normal 
steel  melting  temperatures  slightly  above  the  softening  point 
of  the  brickwork.  In  such  circumstances  the  heat  need  not 
be  rapidly  conducted  from  the  inner  surface,  and  the  walls  may 
be  usefully  thickened  without  in  any  way  impairing  the  life  of 
the  lining,  which  is  then  solely  dependent  upon  other  chemical 
and  physical  influences  described  above.  The  chemical  in- 
fluences are  those  already  mentioned  as  being  harmful  and 
destructive  to  acid  refractory  materials. 

(iii)  The  dimensions  of  the  furnace  body,  which  to  a  greater 
extent  affect  the  radiation  loss  of  small  furnaces,  will  depend 
upon  the  necessary  hearth  capacity,  the  power  input,  the  dis- 
tance of  the  lining  from  the  arc  zones,  and  the  thickness  of  the 
lining. 

For  furnaces  of  small  capacity,  such  as  three  tons  and  under, 
it  is  customary  to  make  the  body  as  small  as  possible,  and  for 
this  purpose  to  reduce  the  thickness  of  walls  and  their  distance 
from  the  arc  zones  to  a  minimum.  This  can  be  understood,  if 
it  is  realised  that  a  small  increase  in  the  outer  diameter  of  a 
small  furnace  means  a  considerable  increase  in  the  ratio  of 
the  radiating  surface  to  the  internal  hearth  capacity. 

The  best  overall  dimensions  are  therefore  based  on  a  com- 
promise between  radiation  loss  and  the  life  of  the  lining.  In 
some  types  the  life  of  a  lining  is  sacrificed  for  the  sake  of 
thermal  efficiency,  and  it  then  becomes  very  questionable 
whether  the  small  thermal  gain  compensates  for  the  cost  of 
constant  repairs,  especially  in  cases  where  the  radiation  loss 
under  the  worst  conditions  is  small  compared  to  the  useful 
energy  input. 

Electrical  Conditions. — The  conversion  of  electrical  energy 
into  heat  by  the  formation  of  an  arc  takes  place  in  small  zones, 
so  that  the  heat  is  developed  at  a  very  intense  temperature^ 
said  to  exceed  3400°  C.  It  is  important  that  the  furnace  charge 
alone  is  submitted  to  such  intense  heat,  a  condition  that  is  only 
possible  when  the  arcs  are  shielded  from  the  lining  either  by 
unmelted  metal,  or  by  the  electrodes  themselves.  A  short  low 
voltage  direct  arc,  shielded  by  a  large  electrode,  produces  the 
least  effect  on  the  lining,  whereas  long  high  voltage  arcs,  strik- 


FEATURES  AND   PRINCIPLES   OF  FURNACE   DESIGN          221' 

ing  from  small  pointed  electrodes,  lead  to  maximum  erosion. 
During  the  greater  part  of  the  melting  down  period,  the  arc 
will  be  partially  surrounded  by  a  wall  of  unmelted  scrap  and 
entirely  shielded  from  the  lining.  Long  arcs  are  permissible 
under  these  conditions,  and  are  in  fact  preferable.  For  this 
reason  it  is  now  customary  to  provide  means  for  altering  the 
arc  voltage  at  will,  which  enables  a  high  voltage  long  arc  to  be 
used  during  the  melting  period,  and  a  low  voltage  short  arc  at 
times  when  the  walls  and  roof  are  exposed. 

The  choice  of  arc  voltage  is  a  matter  of  divided  opinion, 
some  designers  preferring  a  much  higher  voltage  than  others, 
owing  to  the  various  advantages  to  be  gained.  The  question 
will  be  better  understood  by  enumerating  the  various  advantages 
and  disadvantages  of  short  and  long  arcs. 

Low  Voltage  Arcs. — Short  low  voltage  arcs  are  in  closer  con- 
tact with  the  charge,  and  the  heat  is  therefore  better  absorbed 
by  the  metal  and  slag,  which  favours  a  longer  life  of  lining  and 
improves  the  thermal  efficiency.  Certain  slag  reactions,  notably 
those  produced  by  a  reducing  basic  slag,  are  better  promoted  by 
a  large  and  well-shielded  arc  zone,  as  these  reactions  are  favoured 
by  high  arc  temperatures.  According  to  modern  practice,  low 
arc  voltages  vary  between  60  and  40  volts,  the  higher  figure  in 
this  range  being  used  for  melting  and  the  lower  during  refining 
operations.  Low  arc  voltages  involve  no  risk  of  serious  injury 
to  the  furnacemen,  as  the  maximum  open  circuit  line  voltage, 
corresponding  to  three-phase  arcs  (star  connected)  of  60  volts 
each,  will  usually  amount  to  only  about  110  volts ;  in  the  case  of 
two-phase  furnaces  the  open  circuit  voltage  would  be  still  less 
for  similar  arc  voltages.  When  working  with  acid  slags,  which 
have  a  high  electrical  resistance,  the  arc  voltage  should  never 
be  less  than  60. 

Since  the  heat  developed  by  any  arc  is  proportional  to  the 
product  of  current  and  voltage,  then  for  the  same  power  input 
a  short  low  voltage  arc  will  require  a  proportionate  increase  of 
current  (assuming  the  same  power  factor);  this  necessitates 
more  expensive  electrical  plant  and  equipment,  larger  electrodes, 
weaker  roof  lining  construction,  and  a  more  expensive  and 
heavier  electrode  regulating  gear.  The  amount  of  power  dis- 
sipated by  a  short  arc  is  very  sensitive  to  slight  variation  of  its 


222  THE    ELECTEO-METALLUKGY    OF    STEEL 

length,  so  that  the  load  variation  is  likely  to  be  rather  more 
pronounced  when  melting  scrap. 

High  Voltage  Arcs. — Long  high  voltage  arcs  are  injurious 
to  furnace  linings,  and,  owing  to  the  rapid  fusion  of  silica  bricks, 
make  it  difficult  to  carry  out  chemical  operations  which  depend 
upon  the  maintenance  of  a  highly  basic  slag.  Roof  and  wall 
renewals  are  unavoidably  frequent,  which  constitutes  the  main 
objection  to  the  use  of  long  arcs. 

High  voltage  arcs,  whether  direct  or  indirect,  operate  at  110 
volts  or  more  at  full  load,  and  where  considerable  reactance  is 
introduced,  the  arc  voltage  will  increase  to  200  volts  on  open 
circuit  in  many  cases.  Such  a  high  voltage  is  a  source  of 
danger  to  personal  safety,  and  considerable  .care  has  to  be 
exercised  when  performing  any  operation  in  the  immediate 
vicinity  of  the  live  electrode  circuits. 

There  are,  however,  certain  advantages  to  be  gained  by  the 
use  of  high  voltage  arcs.  They  require  less  current  for  the 
same  power,  which  entails  considerable  saving  in  the  cost  of 
electrical  equipment,  as  also  for  the  electrode  regulating  gear. 
Owing  to  the  length  of  a  100  volt  direct  arc,  which,  when  carry- 
ing 3000  amperes,  is  roughly  about  two  and  a  half  to  three 
inches  in  a  hot  furnace,  a  slight  lengthening  or  shortening  will 
have  no  considerable  effect  upon  the  current,  as  the  variation 
of  the  resistance  thus  caused  is  small.  For  this  reason  a  steadier 
load  and  a  better  load  factor  are  more  easy  to  maintain  after 
pools  of  metal  and  slag  have  once  formed  beneath  the  electrodes. 
The  use  of  smaller  electrodes  reduces  electrode  consumption 
per  ton  of  steel,  and  has  an  added  advantage  from  the  point 
of  view  of  roof  construction. 

At  the  present  time  low  arc  voltages  are  generally  favoured, 
but  excellent  all-round  results  have  also  been  obtained  with 
furnaces  melting  on  the  high  voltage  principle  under  favourable 
conditions  of  operation. 

Conductors. — The  design  of  those  portions  of  the  electrical 
circuits  carried  on  the  furnace  body  demands  consideration. 
Heavy  currents  up  to  10,000  amperes  per  phase  are  commonly 
carried,  and  the  magnetic  fields  set  up  by  alternating  currents 
of  such  magnitude  are  considerable.  It  has  been  explained 
how  a  reactive  voltage  is  self-induced  in  such  circuits,  the  effect 


FEATUEES   AND  PRINCIPLES   OF   FURNACE   DESIGN          223 

of  which  is  greatly  magnified  if  iron  or  steel  is  introduced  into 
the  magnetic  field.  Apart  from  increasing  the  reactance  of  the 
circuit,  currents  will  be  set  up  in  the  steel  itself,  and  result  in 
local  heating,  which  in  the  case  of  very  heavy  currents,  becomes 
a  serious  source  of  trouble  in  constructional  design.  Conductors, 
for  this  reason,  are  best  supported  and  guided  by  insulated  bronze 
brackets  bolted  to  the  steel  framework,  and  all  steel  construc- 
tional parts  are  so  designed  to  avoid  making  any  complete  steel 
circuit  around  any  one  set  of  conductors.  Furnace  conductors 
are  usually  built  up  of  heavy  flat  copper  bars,  which,  when 
bolted  together,  are  sufficiently  rigid  to  resist  the  pull  of  the 
flexible  conductors,  either  when  the  furnace  is  being  tilted,  or 
when  the  electrodes  are  being  raised  or  lowered.  Cables  are 
sometimes  used  in  place  of  copper  bars,  but  only  where  they  are 
unexposed  to  high  temperatures,  otherwise  the  copper  wires  in 
time  become  brittle  and  oxidised  and  finally  break.  All  copper 
connections  are  best  made  between  machined  and  tinned  sur- 
faces bolted  together  by  bronze  bolts.  The  terminal  connection 
between  the  copper  bar  conductors  and  the  flexible  cables  is 
often  a  point  of  weakness.  The  simplest  method  of  securing 
a  good  connection  consists  in  clamping  the  cables  firmly  to  the 
conductor  bars  t>y  rigid  bronze  plates,  which  are  grooved  to 
prevent  lateral  slip  on  movement  of  the  cables  ;  these  clamps 
should  be  frequently  examined,  and  any  slack,  caused  by  spread- 
ing of  the  cables  between  the  plates,  taken  up.  When  socketed 
cable  lugs  are  used  great  care  must  be  taken  to  secure  a  perfectly 
sweated  joint,  and  it  is  also  advisable  to  pass  a  set  screw  through 
the  wall  of  each  thimble  or  use  some  other  locking  device  as  a 
further  precautionary  measure.  Under  no  circumstances  should 
sweated  socket  joints  be  used,  unless  only  a  moderate  current 
density  through  the  conductors,  cables,  and  especially  their  joints, 
is  allowed  for.  Local  heating,  when  once  it  occurs,  leads  to 
considerable  trouble  with  this  type  of  connection,  which,  how- 
ever, is  the  most  convenient  and  practicable  if  properly  designed 
and  made. 

The  length  of  conductors  should  be  the  minimum  possible, 
so  as  to  reduce  copper  resistance  losses,  besides  lowering  their 
initial  cost.  Lengthy  conductors,  carrying  6000  amperes  and 
over,  introduce  considerable  reactance  into  the  circuit,  par- 


224  THE    ELECTKO-METALLUKGY   OF    STEEL 

ticularly  when  they  lie  close  to  steelwork.  This  may  be  a  good 
fault  in  some  respects,  but  it  is  usually  preferred  to  cut  down 
this  uncontrollable  reactance  as  far  as  possible,  and  introduce 
external  reactance  coils  of  definite  design.  The  furnace  trans- 
former sub-station  is  best  located  as  near  to  the  furnace  as 
possible,  the  transformers  themselves  being  arranged  at  such 
a  height  that  their  low  tension  terminals  are  at  a  convenient  level 
to  receive  the  flexible  cables  from  the  furnace.  Electrode  holders 
frequently  serve  as  an  integral  part  of  the  load  circuit,  and  are 
then  usually  made  of  bronze  and  rigidly  connected  to  the  copper 
conductor  bars.  Sometimes  they  are  constructed  to  grip  the 
ends  of  the  copper  conductors  firmly  against  the  electrodes,  and 
do  not  themselves  carry  any  current.  Furnace  conductors  are 
always  exposed  to  radiated  heat,  and  are  usually  designed  for  a 
normal  current  density  not  exceeding  1000  amperes  per  square 
inch.  Should  the  conductors  be  very  large  and  their  skin  effect 
considerable,  the  current  density  should  then  be  reduced  propor- 
tionately. With  this  current  density  the  resistance  losses  are  very 
small,  and  the  circuit  connections  on  the  furnace  will  not  be  over- 
heated, if  properly  made.  All  insulating  bushes,  washers  and 
plates  should  be  made  of  a  material  that  remains  unchanged  at 
the  high  temperatures  above  and  around  the  furnace,  as  con- 
stant trouble  is  caused  by  short  circuits  when  the  binding  com- 
position of  the  material  softens. 

Transformer  Capacity. — The  power  capacity  of  steel  furnaces 
is  a  matter  that  very  considerably  influences  the  cost  of  power 
used  per  ton  of  steel  and  the  quantity  of  steel  produced.  It  has 
been  explained  in  Chapter  VI.  how  the  ratio  between  the  power 
wasted  as  radiated  heat  and  the  useful  power  available  for  melt- 
ing will  affect  the  power  consumption  per  ton  of  steel  and  the 
total  output.  Those  arguments  in  favour  of  a  large  ratio  apply 
in  all  cases  where  the  maximum  possible  daily  load  factor  and  out- 
put from  a  given  furnace  is  in  no  way  limited  by  shop  conditions. 
They  are  open,  however,  to  modification  in  cases  where  a  fur- 
nace, having  a  required  holding  capacity,  is  installed  to  operate 
intermittently,  and  therefore  at  a  low  weekly  load  factor. 
Under  such  conditions,  it  may  be  an  advantage  to  reduce  this 
ratio  by  providing  less  power  than  usual :  this  procedure,  of 
course,  increases  the  power  consumption  per  ton  of  steel,  but, 


FEATURES  AND   PRINCIPLES   OF   FURNACE   DESIGN         225 

at  the  same  time,  lowers  the  maximum  demand  or  flat  rate 
payment  to  an  extent  that  will  show  a  small  advantage  on 
balance.  It  has  been  already  shown  (Fig.  77)  how  the  cost  per 
unit  rapidly  rises  under  a  reduced  load  factor,  and  for  this  reason, 
if  the  maximum  demand  or  flat  rate  charges  were  lowered,  with 
a  corresponding  rise  in  monthly  load  factor,  the  net  result 
would  be  a  saving  in  the  power  bill,  notwithstanding  a  small 
increased  power  consumption  and  lengthened  heats.  The 
tendency  is,  therefore,  to  provide  rather  less  power  for  furnaces 
of  a  given  capacity  operating  intermittently  and  at  very  low 
daily  load  factors,  as  this,  besides  reducing  the  power  bill  when 
partly  charged  upon  a  flat  rate  or  maximum  demand  rate,  also 
reduces  the  initial  cost  of  the  furnace  plant. 

Heat  Conversion  of  Electrical  Energy. — Direct  arc  furnaces 
may  be  divided  into  two  distinct  classes,  according  to  the  man- 
ner by  which  the  electrical  energy  is  converted  into  heat. 

I.  Furnaces  in  which  a  conductive  hearth,  or  metallic  con- 
ductors imbedded  in  the  hearth,  become  an  integral  part  of  one 
of  the  load  circuits,  which  is  generally  a  neutral  return  con- 
ductor. 

II.  Furnaces  in  which  the  heat  is  developed  in  direct  arc 
circuits  entirely  independent  of  the  furnace  lining. 

Opinions  are  divided  upon  the  relative  merits  of  these  two 
distinct  types,  and  there  is  no  doubt  that  equally  good  steel  can 
be  produced  with  either.  From  purely  technical  standpoints 
both  designs  have  their  relative  advantages  and  disadvan- 
tages. 

The  several  types  of  conductive  hearth  furnaces  of  small 
capacity,  operated  by  either  two-phase  or  three-phase  current, 
are  provided  with  only  two  electrode  circuits.  In  comparison, 
then,  with  furnaces  of  the  three-phase  three-arc  type  of  similar 
capacity,  the  two-arc  furnaces  should  show  a  reduced  electrode 
consumption,  and  have  the  advantage  of  a  simplified  electrode 
controlling  gear.  A  more  solid  roof  construction  is  also  possible, 
the  roof  being  usually  arched  about  one  horizontal  axis  in  con- 
formity to  the  rectangular-shaped  body  generally  adopted.  A  rec- 
tangular form  certainly  causes  a  greater  radiation  loss  for  a  given 
internal  capacity  than  one  approximating  to  a  sphere,  but  has  an 
advantage  in  reduced  constructional  cost  and  simplicity  of  design. 

15 


226  THE   ELECTEO-METALLUEGY   OF    STEEL 

It  has  been  repeatedly  stated  that  a  certain  amount  of  heat  is 
generated  in  a  conductive  furnace  hearth  by  simple  resistance, 
which  greatly  assists  in  the  manufacture  of  alloy  steels  by 
preventing  the  formation  of  a  frozen  layer  of  metal  on  the  bottom, 
due  to  the  chilling  effect  of  the  cold  alloys  added.  This  bottom 
heating  is  usually  equivalent  to  about  5  per  cent,  to  8  per  cent, 
of  the  full  power  input,  or  27  kw.  to  43  kw.  respectively  for  a 
furnace  operating  under  a  load  of  600  K.V.A.  at  a  power  factor 
of  *9.  If  such  a  hearth  is  homogeneous  and  at  a  uniform  tem- 
perature at  any  horizontal  section,  the  current  density  and  the 
resistance  heating  developed  will  be  uniformly  distributed.  But, 
although  these  hearths  are  generally  constructed  of  layers  of 
material  having  a  progressively  lower  conductivity  towards  the 
top,  it  is  doubtful  whether  the  greater  part  of  the  heat  is 
generated  in  a  region  near  the  bath,  since  the  conductivity  of 
the  top  layers  increases  very  considerably  in  proportion  to  that 
of  the  bottom  layers  at  high  temperatures.  It  is,  therefore, 
impossible  to  know  exactly  in  which  part  of  the  hearth  heat  is 
generated  by  resistance ;  it  is  equally  clear  that  the  total  heat 
generated  is  not  entirely  absorbed  by  the  bath  of  steel,  but  is 
partially  lost  as  a  result  of  increased  radiation  from  the  bottom 
shell  plates.  Should,  however,  the  distribution  of  current 
through  the  hearth  not  be  uniform,  then  there  will  be  more  in- 
tense heating  at  certain  spots  in  the  bottom,  but  this  is  not  likely 
to  occur  if  a  well-constructed  hearth  is  at  a  uniform  temperature, 
as  would  be  the  case  under  a  covering  bath  of  steel.  J.  Bibby 
in  a  paper  contributed  at  a  joint  meeting  of  the  Institution  of 
Electrical  Engineers  and  the  Iron  and  Steel  Institute  in  1919, 
has  gone  further  than  this,  and  gives  his  opinion  that  the  bulk 
of  the  heat  generated  is  dissipated  by  radiation  outside  the 
furnace,  rather  than  any  being  absorbed  by  the  metal.  It  seems 
very  doubtful,  therefore,  whether  the  conductive  hearth  furnace 
can  offer  any  advantage  from  the  point  of  view  of  bottom  heat- 
ing over  the  top  arc  heating  furnace.  Steels  containing  20  per 
cent,  and  more  of  alloyed  metals  can  be  regularly  made  in  the 
latter  type  without  any  steel  chilling  on  the  bottom,  the  natural 
precaution  of  making  small  additions  at  a  time  followed  by 
vigorous  stirring,  which  is  always  necessary  for  mixing  alone, 
being  amply  sufficient  to  secure  this  result. 


FEATURES   A£TD    PRINCIPLES   OF    FURNACE   DESIGN          227 

Bath  circulation  or  auto-mixing  is  another  advantage  claimed 
for  these  furnaces.  Bath  circulation  can  only  be  due  to  either 
heat  convection  currents  or  to  electro-magnetic  effects  set  up  in 
the  bath  itself.  It  is  obvious  that  a  bath  of  steel  heated  by 
direct  arcs  will  be  hottest  at  the  top,  and,  unless  the  bottom  can 
be  heated  to  a  still  higher  temperature,  convection  currents 
cannot  possibly  be  set  up.  Electro-magnetic  circulation  can  in 
no  way  be  due  to  the  result  of  magnetic  fields  dependent  upon 
the  high  permeability  of  iron,  which  becomes  non-magnetic 
above  about  750°  C.  Weak  magnetic  fields  of  varying  intensity 
and  polarity  are,  however,  set  up  by  fluid  conductors  carrying 
heavy  alternating  currents,  and  it  is  then  theoretically  possible 
for  attraction  and  repulsion  between  different  parts  of  a  bath  of 
metal  to  be  caused  by  current  traversing  it  in  different  directions. 
The  mutually  acting  magnetic  forces  induced  by  solid  conductors 
are  visibly  displayed  by  the  movement  of  neighbouring  cables 
of  different  phases  carrying  very  heavy  currents,  but  it  is  diffi- 
cult to  say  whether  sufficient  forces  are  actually  developed  to 
cause  and  maintain  movement  of  heavy  masses  of  molten 
steel  as  a  result  of  currents  traversing  different  paths  through 
the  bath. 

With  certain  types  of  conductive  hearth  furnaces  consider- 
able difficulty  is  experienced  in  securing  a  conductive  circuit 
through  the  bottom,  when  cold.  In  these  cases  it  is  necessary 
to  use  auxiliary  gas  or  oil  heating,  otherwise  the  furnace  can 
only  be  operated  under  an  unbalanced  and  diminished  load. 
To  obviate  this  difficulty  various  modifications  have  been  intro- 
duced embodying  the  use  of  an  auxiliary  upper  electrode,  which 
is  connected  to  the  conductive  hearth  cables  and  can  be  used  to 
complete  the  load  circuits,  thus  enabling  a  balanced  load  to  be 
applied.  Under  continuous  operation,  however,  no  difficulty 
need  be  anticipated  through  failure  of  the  hearth  to  conduct  the 
full  circuit  current  a  few  minutes  after  applying  load.  Experi- 
ence has  proved  that  conductive  hearth  furnaces  can  produce 
excellent  results,  but,  at  the  same  time,  the  character  of  the 
hearth,  owing  to  its  use  as  an  electrical  conductor,  is  not  so 
reliable  under  all  conditions  of  service  as  those  constructed  of 
similar  material  but  independent  of  the  electrical  circuits. 

The  main  advantage  of  the  other  class  of  arc  furnaces,  apart 


228  THE   ELECTKO-METALLUBGY  OF   STEEL 

from  the  ease  with  which  they  may  at  all  times  be  heated 
electrically,  lies  in  the  durability  of  the  hearth,  which,  if  properly 
constructed,  should  never  cause  any  metallurgical  or  other 
difficulties  by  premature  and  sudden  failure  under  any  normal 
conditions.  I'he  disadvantages  are  those  due  to  the  increased 
number  of  electrodes  and  their  raising  gear,  higher  electrode 
consumption,  weaker  roof  construction,  and  the  greater  com- 
plexity of  the  load  regulation  of  three-phase  three-arc  circuits. 
These  disadvantages,  of  course,  only  apply  when  comparison  is 
made  with  conductive  hearth  furnaces  having  not  more  than  two 
top  electrodes.  Although  this  comparison  is  more  especially  ap- 
plicable to  direct  arc  furnaces  alone,  yet  practically  the  same 
arguments  apply  when  comparing  conductive  hearth  furnaces 
with  those  of  the  indirect  arc  type. 

Power  Factor. — The  question  of  power  factor  is  of  great 
importance  in  electric  furnace  design,  and  must  be  considered  in 
its  relation  both  to  the  reactance  of  the  load  circuits  and  to  the 
inherent  reactance  of  the  transformers  or  generating  plant. 
According  to  most  power  contracts  the  flat  rate  or  maximum 
demand  rate  charge  is  based  on  a  K.V.A.  and  not  on  a  K.W. 
input,  so  that  payment  is  made  on  a  figure  which  does  not  re- 
present the  true  maximum  rate  of  power  absorbed.  Since  the 
ratio  of  K.W.  to  K.V.A.  is  proportional  to  the  power  factor,  the 
nearer  the  latter  approaches  unity,  the  better  it  will  be  for  the  con- 
sumer. At  the  same  time,  there  is  a  clause  in  most  contracts 
by  which  the  consumer  guarantees  that  the  average  power  factor 
shall  not  be  less  than  '8  or  '85,  so  that  careful  attention  must  be 
given  to  the  design  of  the  transformers  and  the  load  circuits  to 
the  furnace  electrodes.  The  relation  of  power  factor  to  the  capa- 
city of  transformers  for  doing  useful  work  has  already  been  men- 
tioned, and  determines  the  initial  cost  per  K.W.  capacity  of  plant 
installed  in  contradistinction  to  K.V.A.  capacity,  which  does  not 
truly  indicate  capacity  for  doing  useful  work. 

The  objectionable  features  of  heavy  load  fluctuations,  and  the 
extent  by  which  they  may  be  reduced  by  reactance  coils,  has 
been  fully  dealt  with  in  Chapter  IV.  Where,  however,  such 
reactance  coils  are  introduced  into  the  load  circuits,  they  should 
be  designed  so  as  not  to  reduce  the  power  factor  seriously  at  or 
below  normal  full-load  current,  but  only  on  heavy  current  over- 


FEATURES  AND  PRINCIPLES  OF  FURNACE  DESIGN    229 

loads.  Eeactance  coils  designed  to  fulfil  these  conditions  will 
considerably  reduce  heavy  power  or  K,W.  overloads. 

Certain  furnaces  have  been  designed  in  which  the  amount  of 
reactive  resistance  introduced  into  the  load  circuits  is  such  that, 
at  normal  load,  the  power  factor  is  about  '7.  Under  these  con- 
ditions the  reactive  effect  is  so  great  that  on  dead  short  circuit 
the  power  in  K.W.  is  considerably  reduced  below  normal  full 
load,  and  the  normal  full  load  current  only  increased  by  41  per 
cent.  In  this  way  a  practically  automatic  load  control,  resulting 
in  excellent  load  factors,  can  be  obtained  at  the  expense  of  power 
factor. 

Furnace  design,  from  the  point  of  view  of  power  factor,  will 
then  depend  to  a  great  extent  upon  the  limiting  figure  allowed 
by  the  power  companies,  the  increased  initial  outlay  for  larger 
transformers,  and  the  cost  of  electric  energy  as  purchased  on 
the  basis  of  either  K.V.A.  or  K.W.  demand. 

The  power  factor  of  furnaces  will  be  influenced  by  the  nature 
and  relative  disposition  of  the  circuits  between  the  transformer 
terminals  and  the  electrodes.  Any  individual  circuit  carrying 
an  alternating  current  is  influenced  by  the  magnetic  field  set  up, 
but  if  two  or  more  such  circuits  carrying  currents  that  are  out 
of  phase  are  brought  close  to  one  another,  the  effect  of  the 
magnetic  field  set  up  by  one  will  be  partly  counteracted  by  the 
magnetic  field  due  to  the  others  ;  for  this  reason  the  resultant 
reactive  voltage  induced  in  each  circuit  will  be  very  much 
reduced,  and  the  power  factor  considerably  less  affected.  There- 
fore, when  high  power  factors  are  desired  in  furnace  construction, 
it  is  preferable  to  keep  the  several  conductors  close  together,  to 
avoid  a  closed  iron  circuit  around  any  one  set,  and  to  support 
them  as  far  as  possible  away  from  all  steel  parts.  The  length 
of  a  circuit  also  affects  power  factor  by  increasing  the  self- 
induction. 

Mechanical  Features. — Furnace  Mounting. — It  has  been 
previously  indicated  that  provision  must  be  made  for  tilting 
electric  furnaces  owing  to  the  necessity  of  skimming  a  bath  and 
completely  draining  the  furnace  hearth.  The  earliest  types  of 
both  arc  and  induction  furnaces  were  fixed,  but  the  necessity 
for  tilting  and  emptying  furnaces  for  steel  melting  soon  became 
evident. 


230  THE   ELECTRO-METALLURGY   OF   STEEL 

There  are  two  usual  methods  of  mounting  furnace  bodies 
for  tilting : — 

1.  The  furnace  body  is  carried  on  rocker  castings,  which 
either  roll  forward  on  a  flat  base  plate,  or  are  supported  on  sets 
of  rollers   which  allow  the  furnace  to  roll  about  a  horizontal 
axis. 

2.  The  furnace  body  is  provided  with  trunnions  mounted 
on  trunnion  bearings,  and  can  be  tilted  by  hand  or  mechanically 
driven  gearing ;  this  method  is  generally  confined  to  furnaces 
of  small  capacity. 

Tilting  Gear. — When  hydraulic  power  is  available,  tilting 
may  be  effected  by  one  or  two  rams  situated  under  the  rear  side 
of  the  furnace ;  this  is  undoubtedly  the  most  reliable  and  least 
complicated  method  of  tilting. 

Various  mechanical  methods  have  been  used,  none  of  which 
can  be  said  to  give  entire  satisfaction.  The  tilting  bar  may 
consist  of  a  heavy  screw,  fed  forward  or  backwards  by  a  rotating 
nut,  or  of  a  straight  or  curved  rack  engaged  by  a  pinion  ;  in  the 
latter  case  the  rack  is  fixed  to  the  furnace  body  concentric  with 
the  rocker  castings  supported  on  roller  mountings.  The  screw 
and  nut  method,  which  is  almost  universal  in  Great  Britain,  is 
likely  to  give  trouble  through  failure  of  the  screw  thread  or  ball 
races,  unless  the  gearing  is  carefully  cleaned  and  greased  at 
frequent  intervals ;  this,  however,  is  often  neglected  as  the 
tilting  gear  is  usually  in  a  position  that  is  not  easy  of  access. 
A  rocking  arm  or  connecting  link  is  also  used  for  tilting ;  in 
this  case  a  plain  tilting  bar  is  attached  by  a  swivel  joint  to  the 
furnace  body,  and  connected  at  the  other  end  to  a  crank,  which 
is  made  to  slowly  revolve ;  this  method  is  very  simple,  and  has 
proved  most  satisfactory  for  small  furnaces  of  2  tons  capacity 
and  under. 

In  the  case  of  any  electrically  driven  tilting  gear,  limit 
switches  should  be  provided  to  prevent  the  furnace  being  tilted 
too  far  either  way,  which  might  cause  disengagement  of  the 
tilting  bar  or  strain  on  the  tilting  mechanism. 

Electrode  Regulating  Gear. — The  gearing  used  for  adjusting 
the  electrodes  is  operated  either  electrically  or  by  hand,  pro- 
vision being  usually  made  for  operating  by  either  means  at  will, 
by  introducing  a  simple  clutch  device.  Various  methods  of 


FEATURES  AND   PRINCIPLES   OF  FURNACE   DESIGN          231 

gearing  have  been  employed,  which  utilise  either  a  rack  and 
pinion  drive,  a  nut  and  screw,  feed,  or  a  simple  rope  and  winch 
hoist. 

The  downward  movement  of  any  electrode  should  only  be 
possible  so  long  as  the  full  weight  of  the  electrode  and  its 
mounting  is  carried  by  the  lifting  gear,  and,  as  soon  as  resist- 
ance is  offered  by  the  charge  or  furnace  bottom  to  further 
movement  of  the  electrode,  the  mechanical  gearing  should  be 
thrown  out  of  action ;  in  this  way  no  excessive  strain  can  be 
thrown  on  to  the  gearing.  The  same  applies  to  the  limit  of 
upward  travel,  and  for  this  purpose  limit  switches  and  clutches 
are  also  used.  Power  driven  gearing  is  frequently  operated  in 
conjunction  with  automatic  regulators,  and  is  then  designed 
with  a  sufficient  braking  action  to  prevent  any  tendency  of  the 
motors  to  over-run,  which  would  cause  incessant  hunting. 
The  tendency  to  hunt  is  more  pronounced  where  a  rack  and 
pinion  lifting  gear  is  used,  but,  on  the  other  hand,  a  screw  and 
nut  feed  is  more  liable  to  failure  through  troubles  arising  from 
wear  of  the  screw  threads.  Electrode  raising  mechanisms  are 
always  exposed  to  heat  and  dirt,  and  should,  therefore,  be 
heavily  constructed  and  enclosed  as  far  as  possible.  Electrically 
driven  gearing  is  necessarily  heavy  and  cumbersome,  and 
difficult  to  operate  manually  owing  to  its  low  mechanical 
efficiency,  especially  when  using  heavy  electrodes.  Air  pressure 
has  also  been  employed  for  adjusting  light  electrodes,  but  has 
not  been  developed  to  any  extent.  Hydraulic  control  is  now 
being  introduced  in  place  of  electrically  driven  gearing,  and  is 
being  satisfactorily  developed  for  automatic  regulation. 

The  raising  gear  for  all  electrodes  may  be  mounted  together 
on  one  side  of  a  furnace  shell,  or  may  be  divided  and  attached 
to  two  or  more  sides.  When  this  latter  arrangement  is  adopted 
the  raising  gear  is  always  set  to  one  side  of  the  plane  of  tilting, 
so  that  the  furnace  may  be  provided  with  pouring  spouts  and 
charging  dpors  both  back  and  front ;  this  is  certainly  an  ad- 
vantage as  all  skimming  operations  can  be  performed  over  one 
spout,  while  the  pouring  spout  is  reserved  for  casting. 

When  the  several  raising  gears  are  set  side  by  side,  they  are 
either  attached  to  the  back  of  the  furnace  or  to  one  side  of  the 
plane  of  tilting  so  as  to  utilise  the  double-spout  construction. 


232  THE   ELECTRO-METALLURGY  OF   STEEL 

Electrode  Holders. — There  is  considerable  variation  in  the 
construction  and  design  of  electrode  holders,  which  may  be 
roughly  classified  according  to  whether  the  holder  (i)  is  itself 
the  conductor,  (ii)  clamps  the  conductor  to  the  electrode  and 
carries  its  weight,  or  (iii)  acts  as  a  support  for  some  independent 
clamping  device  which  is  really  an  integral  part  of  the  whole. 

A  holder  belonging  to  the  first  class  should  be  made  of  metal 
of  high  conductivity,  be  water-cooled  or  specially  designed  for 
air  cooling,  and  possess  sufficient  flexibility  to  permit  of  rapid 
opening  or  closing  with  minimum  risk  of  fracture.  A  maximum 
degree  of  flexibility  has  been  obtained  by  hinging  two  portions 
of  a  holder  together ;  the  rigid  portion  is  firmly  fixed  to  the 
electrode  arm,  the  hinged  portion  being  thus  alone  free  to  move. 
Such  holders  are  generally  made  of  bronze,  but  steel  has  also 
been  used  successfully. 

The  same  conditions  apply  to  holders  of  the  second  class, 
only  in  this  case  there  is  no  necessity  for  using  a  gun  metal  or 
bronze  of  high  conductivity,  since  the  holder  itself  is  not  called 
upon  to  carry  current  to  the  electrode.  In  the  case  of  the  third 
class  the  copper  plate  conductors  are  gripped  to  the  electrode 
by  an  independent  flexible  clamp,  which  is  supported  by  a  fixed 
annular  collar  fastened  to  the  movable  carriage  or  gallows  arm. 
The  flexible  clamp  in  this  case  is  only  under  lateral  tension  and 
vertical  compression,  and  is  not  subjected  to  any  bending  or 
twisting  forces.  The  supporting  collar,  which  actually  carries 
the  weight  of  the  electrode,  is  not  required  to  open  and  close, 
so  that  its  construction  can  be  greatly  simplified.  This  division 
of  a  holder  of  the  first  class  into  three  distinct  parts  certainly 
simplifies  the  construction  of  each  part  individually,  but  renders 
the  whole  less  compact. 

A  perfect  holder  has  yet  to  be  designed  which  will  combine 
flexibility,  strength,  electric  conductivity,  and  rapid  means  of 
clamping,  and  at  the  same  time  preserve  these  characteristics 
under  all  normal  conditions  of  working.  The  use  of  graphite 
electrodes  greatly  simplifies  the  construction  of  holders,  and  the 
required  degree  of  flexibility  is  not  sacrificed  to  the  same  extent 
as  for  large  diameter  amorphous  electrodes  where  it  is  necessary 
to  have  great  strength  and  rigidity. 

Furnace  Doors. — A  furnace  door  of  good  design  should  com- 


FEATURES  AND   PRINCIPLES   OF   FURNACE   DESIGN          233 

bine  as  far  as  possible  the  following  features :  (a)  It  should  be 
tight-fitting  but  allow  the  free  escape  of  gases  under  pressure 
from  the  furnace  interior ;  (b)  it  should  be  capable  of  easy  and 
rapid  movement;  (c)  it  should  be  possible  to  open  it  slightly 
for  inspection  purposes ;  (d)  it  should  prevent  undue  loss  of 
heat  through  the  door  opening  in  the  furnace  lining. 

The  simple  lift-up  door  which  fulfils  all  these  conditions  has 
been  severely  criticised,  but,  if  proper  care  is  taken  to  keep  the 
furnace  door  jambs  in  good  condition,  the  electrode  consumption 
due  to  in-draught  of  air  is  not  materially  increased.  The 
various  designs  of  close-fitting  swinging  doors  are,  from  a 
mechanical  point  of  view,  quite  satisfactory,  but  are  not  suitable 
as  inspection  and  working  doors,  since  they  cannot  be  slightly 
opened  for  spoon  sampling  and  other  manipulative  operations. 

At  least  one  door  opening  should  be  large  enough  for  the  re- 
moval of  a  full-diameter  piece  of  electrode,  and  all  others  large 
enough  for  the  purpose  of  charging  uniformly  and  fettling. 


CHAPTEK  XII. 

MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES. 

THE  several  types  of  electric  steel  furnaces  now  in  use  may 
best  be  studied  in  the  order  in  which  they  have  been  suc- 
cessively introduced.  In  this  way  the  introduction  of  novel 
features  peculiar  to  any  particular  design  will  be  more  readily 
understood.  It  is  also  necessary  to  divide  electric  arc  furnaces 
into  two  distinct  classes  : — 

(a)  Indirect  arc  furnaces. 

(b)  Direct  arc  furnaces. 

Indirect  Arc  Furnaces. — This  class  includes  all  arc  furnaces 
in  which  the  arc  strikes  between  electrodes,  so  that  the  furnace 
charge  is  entirely  independent  of  the  arc  circuits  and  receives 
heat  by  radiation  and  reflection  alone.  This  type  was  originated 
by  Siemens,  whose  furnace  is  illustrated  in  Fig.  2. 

Stassano  Furnace. — Stassano  was  the  first  to  use  a  single 
indirect  arc  for  metallurgical  purposes  conducted  on  a  com- 
mercial scale,  and  in  1898  built  his  first  furnace,  which  was 
intended  for  the  direct  production  of  steel  from  iron  ore.  This 
furnace  did  not  meet  with  economic  success,  so  that  Stassano 
ultimately  modified  its  construction  for  melting  steel  scrap. 

The  outstanding  feature  of  the  modern  Stassano  furnace 
lies  in  the  mechanical  method  of  mixing  the  molten  or  semi- 
molten  charge  in  order  to  utilize  the  heat  radiated  by  the  arc 
to  the  best  possible  advantage.  There  are  also  several  less  im- 
portant features  embodied  in  this  design,  which  are  nevertheless 
characteristic : — 

(i)  Fixed  orientation  and  inclination  of  the  electrodes. 

(ii)  Special  hydraulic  electrode  regulating  mechanism,  oper- 
ated by  low  pressure  water  circulating  in  cooling  jackets,  which 
carry  the  electrode  holders. 

(234) 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES 


235 


(iii)  The  melting  chamberx  is  lined  with  magnesite  bricks, 
and  assumes  the  form  of  a  hollow  segment  of  a  sphere,  or  of  an 
ellipsoid  for  large  furnaces  (Figs.  102  and  103). 

A  vertical  section  of  the  furnace  as  shown  in  Fig.  103  clearly 
illustrates  these  special  fea- 
tures of  the  construction. 
The  furnace  body  is  pro- 
vided with  trunnions, 
which  rest  on  bearings 
carried  by  a  ring  encircling 
the  furnace  body.  This 
ring  also  carries  trunnions, 
which  are  supported  on 
fixed  pedestal  bearings. 
The  axes  of  the  two  sets 
of  trunnions  are  set  at  90° 

to  each  other,  so  that  the  furnace  is  free  to  swing  in  every  direc- 
tion just  like  a  compass  mounted  in  a  gimbal.  A  pivot  is  fixed 
centrally  to  the  underside  of  the  bottom  plate,  and  is  displaced 


>7 


ffr£3 

« '«  s.C 


FIG.  103. 


to  one  side  of  the  normal  vertical  axis  of  the  furnace  by  a  large 
bevel  wheel,  which  engages  the  pivot  through  an  adjustable 
ball  and  socket  bearing.  The  bevel  wheel  is  axial  to  the  trun. 
nion  ring  and  the  normal  axis  of  the  furnace  when  vertical,  so 


236  THE   ELECTRO-METALLTJKGY   OF   STEEL 

that  rotation  causes  the  pivot  to  describe  a  circle  about  the 
normal  vertical  axis,  and  imparts  an  oscillating  movement  to 
the  furnace  body ;  the  degree  of  oscillation  can  be  easily 
adjusted  by  altering  the  eccentricity  of  the  pivot. 

This  construction  is  a  considerable  departure  from  the  earlier 
types,  in  which  the  furnace  body  slowly  rotated  about  a  slightly 
inclined  axis,  a  design  which  necessitated  the  supply  of  power 
to  the  electrodes  through  rubbing  contacts. 

The  furnace  lining  is  composed  of  magnesite  brick,  which  is 
surrounded  by  a  heat  insulating  backing  of  either  brick  or  special 
refractory  earths,  the  shape  of  the  melting  chamber  being  de- 
signed to  reflect  the  heat  downwards  on  to  the  charge  or  bath. 
The  lining  is  naturally  exposed  to  a  very  intense  heat,  and  for 
this  reason  only  magnesite  brick  can  be  successfully  used,  and 
then  only  when  certain  brands  are  available,  which  not  only 
stand  up  to  the  intense  temperature,  but  resist  "  spalling  "  to  a 
most  marked  degree.  A  single  charging  door  is  provided,  to- 
gether with  a  small  inspection  hole,  both  of  which  are  closed 
when  highly  reducing  conditions  are  required.  A  closed  tap- 
hole  is  used  and  any  slag  skimming  has  to  be  done  through  the 
charging  door. 

Two  furnaces  of  the  above  described  type  were  installed  in 
the  north  of  England  for  the  manufacture  of  light  intricate 
steel  castings.  They  were  of  1  ton  capacity  and  designed  for 
three-phase  operation,  each  being  equipped  with  a  300  K.V.A. 
transformer  supplying  three-phase  current  at  a  line  voltage  of 
either  150  or  100  volts.  The  high  voltage  was  used  more 
especially  for  melting,  and  the  low  after  fusion  of  the  charge 
was  complete.  Such  high  voltage  arcs  are  always  considerably 
drawn  out,  especially  in  a  hot  furnace,  and  care  has  therefore 
to  be  exercised  when  charging  in  fresh  scrap,  so  as  not  to  break  an 
arc  and  thus  interrupt  the  proper  electrical  conditions.  The 
method  of  electrode  mounting  used  on  these  particular  furnaces, 
as  shown  in  Fig.  103,  enabled  the  electrodes  to  be  rapidly  re- 
moved or  adjusted  in  their  holders.  Three  cylindrical  water 
jackets,  fixed  to  the  furnace  shell  at  an  inclined  angle  and  con- 
verging to  a  common  centre  at  angles  of  120°,  served  as  guide 
boxes  for  the  electrodes.  Each  jacket  also  carried  two  pro- 
jecting guide  rods  upon  which  the  base  of  the  holder  was  free 


MODERN   TYPES   OF  ELECTRIC   STEEL  FURNACES.  237 

to  slide,  the  rods  being  so  spaced  that  the  axis  of  the  holder  was 
central  to  the  cooling  jacket.  The  holder  was  indirectly  con- 
nected to  the  end  of  a  piston  rod,  operating  in  a  small  cylinder 
fastened  to  the  underside  of  the  cooling  jacket,  and  was  thus 
capable  of  axial  movement  and  rapid  removal.  These  furnaces 
of  1  ton  capacity  are  reputed  to  have  made  80  to  85  heats 
before  requiring  to  be  relined,  and  used  about  1100  units  for 
each  heat. 

The  furnace  load  is  controlled  with  the  aid  of  ammeters 
which  indicate  the  current  flowing  through  each  electrode 
circuit,  the  electrodes  being  moved  either  inwards  or  outwards 
until  the  current  flowing  through  each  is  at  the  desired  value. 
If  the  arc  between  one  pair  of  electrodes  is  shorter  than  either 
of  the  other  arcs,  then  the  current  flowing  through  either  elec- 
trode of  that  pair  will  be  greater  than  the  current  flowing 
through  the  third.  Balance  of  current,  therefore,  is  only 
possible  when  the  arc  lengths  are  equal,  and  when  the  electrode 
tips  form  the  apices  of  an  equilateral  triangle.  The  arcs  them- 
selves are  mesh  connected,  so  that  the  current  flowing  through 
each  arc  equals  the  line  current  -f-  1*73. 

If  A  is  the  current  flowing  through  each  electrode  circuit 
and  V  equals  the  line  voltage,  then  the  power  can  be  calculated 
from  the  equation — 

w_AxVx3x  power  factor 
1-73  x  1000 

Rennerfelt  Furnace. — The  outstanding  feature  of  this  fur- 
nace lies  in  a  special  arrangement  of  the  electrodes,  whereby 
the  arcs  are  forced  to  take  the  shape  of  a  flame  that  is  strongly 
deviated  downwards  in  the  form  of  an  arrow  head.  The  heat 
is  in  this  way  more  concentrated  on  those  zones  where  it  is 
required  for  melting  and  refining  purposes,  and,  at  the  same 
time,  the  roof  and  upper  walls  of  the  lining  are  not  exposed  to 
the  same  intense  heat  of  uncontrolled  indirect  arcs  that  always 
have  a  natural  tendency  to  flame  upwards.  Unlike  other  in- 
direct arc  furnaces  there  is  also  a  shading  effect  from  a  vertical 
electrode,  which  is  to  some  extent  comparable  to  that  of  a  direct 
arc  furnace.  The  arc  zones  can  also  be  moved  in  a  vertical 
plane,  so  that  their  distance  from  the  charge  can  be  kept  con- 
stant as  melting  proceeds. 


238 


THE   ELECTRO-METALLURGY    OF    STEEL 


Electrical  Design. — The  furnace  as  generally  constructed 
operates  on  a  low  tension  two-phase  system,  the  current  being 
conveyed  to  the  melting  chamber  by  three  circuits  connected  to 
adjustable  electrodes.  One  circuit  serves  as  a  neutral  return  for 
the  current  flowing  through  the  two  phases,  and  is  connected 
to  a  vertical  electrode  passing  centrally  through  the  roof.  The 


outer  terminals  of  each  phase  are  connected  to  horizontal  elec- 
trodes, the  axes  of  which,  together  with  that  of  the  neutral 
electrode,  lie  in  the  same  vertical  plane.  The  arcs  strike  between 
the  tips  of  the  horizontal  and  vertical  electrodes,  and  are  de- 
flected downwards  by  the  resultant  magnetic  effect  of  the  fields 
set  up  by  each  arc. 

A  complete  diagram  of  the  power  supply,  furnace,  and  in- 
strument connections  is  shown  in  Fig.  104.     The  power  supply 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     239 

is  three-phase,  as  indicated  by  the  three  line  wires  L,  which  are 
brought  into  an  automatic  tripping  oil  switch  1.  The  high 
tension  cables  then  pass  to  a  set  of  choking  coils  2,  each  of 
which  is  divided  into  two  unequal  parts  in  the  ratio  of  1  to  2 ; 
circuit  breakers  enable  either  portion  to  be  short-circuited,  so 
that  three  different  values  of  choking  effect  may  be  obtained. 
These  choking  coils  are  frequently  introduced  into  the  low  tension 
circuits.  The  high  tension  current  is  transformed  down  to  a 
suitable  voltage  by  a  Scott-connected  group  of  transformers  4, 
primary  tappings  and  selector  switches  6  being  provided  for 
secondary  voltage  variation.  Auto-transformers  installed  on  the 
secondary  side  of  the  power  transformers  have  also  been  used 
for  this  purpose.  The  low  tension  circuits  Ph.  I.  and  Ph.  II. 
are  shown,  together  with  the  neutral  return  conductor  Ph.  I. 
and  Ph.  II.  The  current  transformers  8  operate  the  various 
controlling  instruments  and  the  automatic  regulators,  if  used. 

The  usual  voltages  available  between  the  horizontal  outer 
electrodes  and  the  vertical  neutral  are  80  and  100.  When 
choking  coils  are  used  on  the  secondary  side,  the  voltage  across 
each  phase  is  about  150,  which  allows  for  a  considerable  arc 
voltage  drop  on  normal  full  load.  The  load  is  regulated  by 
moving  the  two  side  electrodes  either  towards  or  away  from  the 
vertical  neutral  electrode,  which  is  always  so  adjusted  that  the 
tips  of  all  three  are  in  line.  Electrode  adjustment  is  effected 
either  by  hand  or  automatic  control.  When  the  load  is  equally 
balanced  between  the  two  arcs,  the  current  through  the  neutral 
is  1'41  times  the  current  flowing  through  each  phase.  The  side 
electrodes  are  capable  of  being  tilted  downwards,  so  that  it  is 
also  possible  to  strike  two  entirely  distinct  direct  arcs  on  to 
a  molten  charge,  provided  the  neutral  electrode  is  likewise  in 
contact  with  the  slag  or  dips  into  it. 

Three  sets  of  bus  bars  are  brought  out  horizontally  from  the 
transformer  house  at  a  point  well  above  the  furnace,  flexible 
cables  being  then  employed  for  connecting  these  bus  bars  to  the 
three  electrode  holders.  The  transformer  ratings  for  various 
furnace  capacities  are  given  in  the  following  table : — 


240         THE  ELECTRO -METALLURGY  OF  STEEL 

Furnace  Capacity.  Transformer  Rating. 

4  cwts 75  K.V.A. 

7      „  .         ...  125       „ 

15      „  ....  250       „ 

14  tons 400 

3-3*     „  .         .  •       .         .  800       „ 

4-44     „  .  1000       „ 

The  power  factor  is  normally  about  '90  at  normal  full  load 
which  allows  for  sufficient  circuit  reactance  to  prevent  very 
heavy  fluctuations  or  short-circuit  currents. 

Structural  Features. — The  furnace  body  of  the  most  modern 
type  is  built  in  the  form  of  a  vertical  cylinder  with  a  flat  bottom, 
and  is  covered  by  a  detachable  circular  roof.  The  furnace  is 
mounted  either  on  trunnions  or  on  rockers  to  permit  tilting, 
which  is  done  by  hand  in  the  case  of  the  smaller  sizes  up  to  about 
l^  tons  capacity.  A  half  section  front  elevation  of  a  trunnion 
mounted  furnace  is  shown  in  Fig.  105.  The  electrodes  pass 
through  cylindrical  cooling  jackets,  which  are  pivoted  on  brackets 
fastened  to  the  furnace  shell.  These  jackets  are  also  rigidly 
connected  to  the  electrode  carrying  frame,  which  can  be  tilted 
by  means  of  the  hand  wheels  shown  on  the  extreme  sides  of  the 
drawing.  The  electrode  holders  are  adjusted  by  a  nut  and  screw 
feed,  driven  either  by  hand  or  motor  as  shown.  The  motors 
are  either  fixed  under  the  carriers  or  on  brackets  bolted  to  the 
furnace  shell. 

The  rectangular  furnace  (Fig.  106)  has  been  designed  for 
capacities  of  5  tons  and  over,  and  embodies  a  complete  duplica- 
tion of  the  low  tension  furnace  circuits  operating  in  parallel. 
A  furnace  of  this  type  of  4  to  5  tons  capacity  with  a  power 
input  of  750  K.V.A.  has  been  in  use  for  two  years  at  the  works 
of  Stridsberg  and  Biorck,  at  Trollhatten,  for  making  high  class 
carbon  steels. 

Furnace  Lining. — Acid  and  basic  linings  are  both  employed, 
the  walls  being  in  either  case  14  inches  thick,  which  includes  a 
44  inch  backing  of  fire-brick.  A  section  through  a  rectangular 
basic  lined  furnace  is  shown  in  Fig.  107.  The  bottom  is  covered 
with  two  courses  of  fire-brick,  above  which  is  laid  a  single  course 
of  magnesite  bricks  placed  on  edge  :  the  fire-brick  is  shown 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     241 

stepped  up  towards  the  sides  and  carried  to  the  top  as  a  backing 
to  the  magnesite  and  silica  wall  bricks.     The  magnesite  bricks 


are  also  stepped  up  and  carried  to  a  point  a  few  inches  above 
the   slag  line,    and   from   there   the   walls   are   built   of  silica 

16 


242 


THE    ELECTEO-METALLUEGY   OF    STEEL 


bricks.  The  hearth  is  built  of  a  mixture  of  calcined  mag- 
nesite  and  basic  slag,  which  can  be  sintered  in  layers  by 
the  heat  of  the  arcs.  In  the  modern  circular  form  of  body 
the  walls  are  built  up  to  the  top  of  the  steel  shell,  and  form  a 
seating  for  a  circular  domed  roof,  which  is  lined  with  special 
9-inch  silica  bricks ;  the  sintered  hearth  is  also  about  double  the 
thickness  of  that  shown  in  Fig.  107.  In  the  case  of  acid  lined 
furnaces,  silica  brick  and  ganister  are  used  in  place  of  magnesite 
brick  and  the  sintered  basic  hearth  mixture. 

Electrodes, — Graphite    electrodes   are    preferred,    and     are 


generally  loaded  up  to  150  to  220  amps,  per  sq.  in.  Small 
sized  electrodes  have,  however,  been  loaded  as  high  as  400  amps, 
per  sq.  in.  The  vertical  neutral  electrode  is  larger  in  diameter 
than  the  side  electrodes,  owing  to  the  heavier  current  carried. 
The  electrode  consumption  has  been  carefully  ascertained  in 
terms  of  Ibs.  consumed  during  each  hour  of  operation  under 
definite  conditions.  This  is  certainly  a  convenient  and  accurate 
way  of  expressing  electrode  consumption,  which  enables  the 
consumption  per  ton  of  steel  to  be  approximately  estimated  for 
intermittent  or  continuous  operation. 

The  following  table  gives  actual  figures  of  electrode  consump- 


MODEKN  TYPES  OF  ELECTKIC  STEEL  FUENACES 


243 


tion  for  furnaces  operating  with  graphite  electrodes  of  various 
diameters  for  the  manufacture  of  tool  steel  and  castings  : — 


Diameter  of  the  Side 
Electrodes. 

Diameter  of  Vertical 
Electrode. 

Lb.  per 
Hour. 

Period 
Averaged. 

1£  in.  graphite 

1£  in.  graphite 

•5 



2 

3 

•66 

1224  hours 

3 

3 

1-5 

several  months 

3i 

4 

2-36 

— 

5 

5 

2-2 

480  hours 

4 

5 

i 

1-65 

130      „ 

2  sets  of  5 

2  sets  of  6 

6-4 

130      „ 

The  Kennerfelt  furnace,  apart  from  its  use  for  making  tool 
steel  and  castings  has  been  employed  for  melting  ferro-alloys 
and  special  grades  of  pig-iron.  It  has  found  considerable  favour 
in  the  United  States  of  America  and  Scandinavia,  but  is  at 
present  not  widely  known  in  Great  Britain. 

Furnace  Operation. — The  furnace  may  be  preheated  simply 
by  means  of  the  free  burning  arcs,  which  can  be  kept  at  any 
desired  distance  from  the  bottom.  The  usual  practice  is  to 
start  melting  a  cold  charge  of  scrap  by  means  of  the  free 
burning  arcs,  which  are  gradually  lowered  as  the  charge  melts. 
When  the  charge  has  been  completely  melted,  and  it  is  desired 
to  obtain  strongly  reducing  slag  conditions,  the  side  electrodes 
are  tilted  downwards  until  direct  arcs  strike  on  to  the  slag,  the 
neutral  electrode  being  at  the  same  time  lowered  to  make  contact 
with  the  bath.  Under  these  conditions  the  ordinary  deoxidising 
and  desulphurising  carbide  slag  can  be  maintained. 

Since  the  charge  is  quite  independent  of  the  arc  circuits,  it 
follows  that  the  load  is  not  so  subject  to  fluctuation  during  the 
melting  period  as  in  furnaces  of  the  direct  arc  type.  The 
circuit  reactance  prevents  heavy  overloads  when  striking  the 
arc,  and  tends  to  steady  the  current,  especially  when  starting  to 
heat  up  a  cold  furnace.  The  chief  difficulty  of  manipulation  lies 
in  charging  the  scrap.  Great  care  must  obviously  be  taken  to 
prevent  the  side  electrodes  arcing  on  to  the  charge  when  they 
are  being  tilted  downwards  as  melting  proceeds ;  the  same  diffi- 
culty applies  when  charging  scrap  into  the  bath.  Heavy  small 


244  THE   ELECTRO-METALLURGY   OF    STEEL 

scrap   will   give   less    trouble   than   bulky  light   scrap,    which 
requires  more  constant  feeding. 

The  Kennerfelt  furnace  has,  so  far,  only  been  used  for  melt- 
ing cold  scrap  charges  up  to  5  tons  in  weight.  Larger  units 
are  being  developed,  which  will  be  more  suitable  for  refining 
liquid  steel. 

DIRECT  ARC  FURNACES. 

Heroult  Furnace. — In  this  furnace  the  principle  of  direct  arc 
heating  was  first  commercially  applied  to  the  metallurgy  of  steel. 
Direct  arc  furnaces  had  been  used  for  many  years  before  the 
introduction  of  Heroult's  modified  form,  and  were  always  pro- 
vided with  a  carbon-lined  bottom,  which  was  connected  to  one 
terminal  of  a  single-phase  power  circuit.  It  was  essential  to 
eliminate  this  carbon- conducting  bottom  to  prevent  carbon 
absorption  by  the  steel,  and  this  Heroult  accomplished  by 
splitting  the  single  direct  arc,  as  hitherto  used,  into  two  direct 
arcs  in  series,  using  the  metallic  charge  to  complete  the  circuit 
between  the  two  arcs.  This  enabled  any  suitable  refractory 
material  to  be  used  for  the  hearth  lining,  and  burnt  dolomite 
was  chosen  for  that  purpose. 

Heroult's  chief  aim  was  to  produce  a  furnace  of  simple 
design,  in  which  the  basic  open  hearth  process  of  steel-making 
could  be  practised  by  merely  substituting  electric  heating  for 
gas.  This  aim  was  actually  realised,  but  it  was  found  too 
expensive  to  use  the  electric  furnace  for  boiling  out  carbon  from 
pig-iron  for  conversion  to  steel.  For  this  reason  its  application 
was  later  confined  to  melting  and  refining  mixed  charges  of 
scrap  iron  and  steel,  in  which  the  carbon  was  not  sufficiently 
high  to  prolong  the  refining  operations.  It  was  also  found  that 
internal  electric  heating  enabled  a  highly  reducing  atmosphere, 
and  consequently  reducing  slag  conditions,  to  be  maintained 
within  the  furnace,  and  this  resulted  in  the  discovery  of  further 
refining  powers  in  the  nature  of  sulphur  and  oxygen  removal. 

A  bottom  metallic  electrode,  imbedded  in  a  refractory  hearth, 
was  also  tried  as  a  substitute  for  the  carbon  bottom  and,  although 
made  the  subject  of  a  Belgian  patent  application  in  1902,  was 
abandoned  in  favour  of  keeping  the  arc  circuits  entirely  in- 
dependent of  the  furnace  lining.  This  early  decision  of  Heroult 


MODEEN   TYPES  OF  ELECTRIC   STEEL   FURNACES  245 

has  been  firmly  upheld  to  the  present  day,  so  that  the  out-stand- 
ing feature  of  the  original  Heroult  steel  furnace  still  remains. 

The  single-phase  furnace  design  was  not  suitable  for  operation 
on  polyphase  systems,  owing  to  the  high  cost  of  motor-generator 
sets  coupled  with  their  poor  electrical  efficiency.  For  this  reason 
the  furnace  was  redesigned  to  operate  on  a  three-phase  supply, 
being  provided  with  three  electrodes  for  striking  star-connected 
arcs  on  to  the  metallic  charge  or  bath,  which  serves  as  a  star  point. 

Features  of  the  Electrical  Equipment. — Three  single-phase 
transformers  are  connected  in  either  delta-delta  or  star-delta 
fashion,  and  with  this  simple  method  of  grouping,  the  trans- 
formers are  identically  the  same,  so  that  it  is  only  necessary  to 
keep  one  spare  in  case  of  emergency.  The  connections  of  the 
primary  windings  can  also  be  made  readily  interchangeable  from 
star  to  delta  or  vice  versa,  when  considerable  variation  of  the 
secondary  voltage  is  desired.  Flexible  cables  are  taken  direct 
from  the  secondary  terminals,  where  the  mesh  connection  is 
made,  to  the  cable  clamps  attached  to  the  furnace  conductor 
bars,  so  that  the  least  possible  Length  of  cable  is  used.  The 
primary  windings  are  always  provided  with  one  or  two  tappings 
for  effecting  voltage  variation  across  the  secondary  circuits,  this 
being  done  by  means  of  special  switches,  as  already  described 
in  Chapter  IV. 

The  usual  line  voltages  employed  for  basic  working  are  84 
and  72  volts  at  normal  full  load,  the  open  circuit  voltage  being 
somewhat  higher  according  to  the  reactance  of  the  circuits. 

For  working  the  acid  process,  a  considerably  higher  line 
voltage  is  necessary,  owing  to  the  high  electrical  resistance  of 
the  siliceous  slag,  and  for  this  purpose  a  line  voltage  of  110  at 
normal  full  load  is  usually  provided,  corresponding  to  an  arc 
voltage  of  approximately  63  volts.  The  transformers  suitable 
for  various  furnace  capacities  are  generally  rated  as  follows : — 

Furnace  Capacity.  Transformer  Capacity  in  K.V.A. 

lOcwts.     .         .         .         200-400 
li  tons-  .        .        .         450-600 

2  „      .-•    V        .  600 

3  „    '•;      \  .         .        600-900 
6-7       „.;         .       1200-1800 

10       „      .      ".         .       1800-2400  for  melting  cold  scrap- 


246  THE    ELECTRO-METALLURGY   OF   STEEL 

The  power  factor  of  the  furnace  load  is  invariably  higher 
than  the  guaranteed  figure  of  '85,  which  some  power  companies 
demand,  and  installations  are  working  for  which  the  average 
monthly  K.V.A.  maximum  demand  is  calculated  on  a  carefully, 
recorded  average  power  factor  of  '90  to  '92. 

The  power  circuits  are  always  designed  with  sufficient 
reactance  to  prevent  very  heavy  current  overloads,  and  when 
reactance  coils  are  introduced  into  the  low  tension  circuits  they 
are  designed  to  produce  only  a  small  reactance  drop  at  normal 
full  load,  wrhich,  however,  rises  very  rapidly  on  overloads.  In 
this  way  the  power  factor  is  hardly  affected  at  normal  full  load 
current. 

The  amount  of  current  flowing  through  each  electrode  is 
indicated  by  an  ammeter,  three  of  which  are  usually  mounted 
on  a  panel  fixed  to  the  back  framework,  just  above  the  electrode 
raising  gear.  The  panel  also  carries  three  lamps,  each  of  which 
is  connected  between  one  set  of  cables  and  a  common  point 
connected  with  the  furnace  hearth.  When  these  lamps  are  of 
equal  brilliancy  it  is  an  indication  of  balance,  since  the  arc 
voltages,  and  therefore  the  current  through  each  electrode,  must 
be  equal  to  produce  this  effect.  The  luminosity  of  each  lamp, 
being  dependent  upon  the  voltage  between  each  electrode  and 
the  furnace  charge,  is  bright  unless  the  electrode  touches  the 
charge,  when  the  lamp  is  extinguished.  In  this  connection  it 
should  be  noted  that  one  electrode  can  be  forcibly  lowered  on 
to  a  charge  of  scrap  without  causing  any  current  to  flow  until 
one  of  the  other  two  completes  the  circuit,  and  for  this  reason  a 
lamp  which  indicates  contact  is  an  exceedingly  useful  accessory 
to  an  ammeter,  and  prevents  breakage  of  fragile  graphite 
electrodes.  With  a  constantly  breaking  load  they  are  also 
most  useful,  as  they  enable  the  electrodes  to  be  rapidly  adjusted 
with  less  risk  of  causing  heavy  overloads  on  again  striking  arcs. 

An  indicating  wattmeter  and  voltmeter  are  usually  mounted 
on  a  separate  panel,  which  is  hinged  to  a  wall  bracket  and  can 
be  swung  outwards  into  a  prominent  position.  The  wattmeter 
connections  are  made  to  current  transformers  placed  in  each 
electrode  circuit,  and  to  the  three  furnace  conductor  bars.  The 
voltmeter  is  arranged  to  indicate  by  suitable  plug  connections 
the  line  voltage  and  any  of  the  three  arc  voltages. 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     247 

Besides  the  foregoing  instruments,  used  for  regulating  the 
furnace  load,  a  graphic  recording  wattmeter  and  an  integrating 
watt-hour  meter  are  generally  installed,  both  operating  off  the 
low  tension  power  transformer  circuits. 

Furnace  Design. — The  modern  furnaces  are  three-phase  and, 
except  for  the  largest  sizes,  the  same  general  design  (Fig.  108) 
is  adopted  for  all. 

The  furnace  body  is  octagonal  to  conform  as  far  as  possible 
to  the  circular  form  of  the  melting  chamber ;  at  the  same  time 
this  shape  simplifies  the  construction  of  the  doors  and  the 
attachment  of  the  electrode  columns  and  raising  gear.  The 
bottom  plate  is  bent  to  a  slight  curve  and  is  also  more  easily 
constructed  than  would  be  the  case  for  a  cylindrical  furnace 
shell.  The  octagonal  form  of  the  shell  for  a  given  holding 
capacity  reduces  the  surface  of  radiation  to  a  minimum.  The 
shell  is  bolted  on  to  two  rocker  castings,  which  roll  forward  on 
a  cast-iron  bed  plate  on  tilting,  these  rockers  being  rigidly 
braced  together  by  two  cast-iron  separators  and  one  steel  casting 
to  which  the  tilting  screw  is  connected.  Three  door  openings 
are  provided,  one  at  each  side  and  one  in  the  front  wall  im- 
mediately above  the  pouring  spout,  so  that  every  part  of  the 
furnace  hearth  is  readily  accessible  both  for  charging  and 
fettling  operations.  The  shell  plates  are  strengthened  at  each 
door  opening  by  a  cast-steel  stiffener  through  which  the  furnace 
doors  are  raised  and  lowered,  the  doors  being  suspended  by 
chains  from  a  rocking  arm,  pivoted  on  an  angle  support  and 
balanced  by  counter- weights. 

The  steel  framework  which  carries  the  electrode  carriages 
and  gallows  arms  consists  of  three  pairs  of  channels,  each  pair 
being  set  with  their  flanges  facing  so  as  to  form  a  long  rectangular 
guide-box  for  two  pairs  of  rollers  attached,  one  at  each  end,  to 
the  steel  electrode  carriage.  The  latter  is  free,  then,  to  move 
up  and  down  between  these  channels  with  only  sufficient  lateral 
movement  to  ensure  ease  of  working.  Two  projecting  lugs  are 
cast  on  the  back  of  each  carriage,  between  which  a  rack,  guided 
by  means  of  straps,  is  free  to  slide.  The  racks  are  meshed  in 
with  pinions  which  are  driven  through  reduction  gearing  and 
strongly  mounted  on  brackets  attached  to  the  back  framework. 
In  the  event  of  the  downward  movement  of  the  electrode  being 


248 


THE   ELECTKO-METALLUEGY   OF   STEEL 


FIG.  108.— 6-Ton  Heroult  Furnace. 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     249 

resisted,  the  rack  will  no  longer  carry  the  weight  of  the  arm, 
and,  by  moving  away  from  the  top  projecting  lug,  opens  a  switch 
which  automatically  stops  the  motors.  This  device  is  a  safe- 
guard against  damage  to  the  electrodes  or  the  raising  mechanism. 
The  steel  electrode  carriages  are  cast  with  short  arms,  to  which 
are  fixed  extension  pieces  of  steel  channel  carrying  the  holders, 
from  which  they  are  carefully  insulated.  The  arms  of  the  two 
outer  carriages  are  slightly  set  inwards,  so  that  the  centre  lines 
of  the  channel  extensions  pass  through  the  centres  fixed  for  the 
electrode  axes  ;  in  this  way  the  three  holders  can  be  made 
identical  and  therefore  interchangeable. 

A  space  is  provided  between  the  back  shell  plate  and  the 
channel-guide  framework,  which  are  rigidly  connected  together 
by  steel  plates  at  each  end  so  as  to  form  a  narrow  rectangular 
chamber.  The  conductor  bars  pass  down  through  this  chamber, 
and  are  guided  by  insulated  gun-metal  boxes,  which  bridge  across 
both  the  top  and  bottom.  With  this  construction  there  is  no 
complete  iron  circuit  surrounding  any  one  phase,  and  the 
heating  effect  of  eddy  currents  and  a  reduced  power  factor  are 
avoided. 

The  electrode  holder  is  built  in  two  halves  hinged  together 
and  water  cooled,  the  water  connection  from  one  half  to  the 
other  being  made  by  means  of  a  short  loop  of  copper  tube.  A 
lug  of  ample  dimensions  is  cast  on  to  the  rigid  half  of  the  holder, 
and  is  machine-faced  for  connection  to  the  conductor  bars. 
These  bars  are  firmly  held  in  place  by  insulated  gun-metal 
brackets  attached  to  the  upper  side  of  the  electrode  carriage. 
Water  circulating  pipes  to  and  from  the  holders  are  clipped  to 
the  conductor  bars,  and  terminate  alongside  the  cable  clamps. 

The  roof  frame  is  circular,  and  when  bricked  up  rests  upon 
the  body  lining,  four  lugs  being  riveted  to  the  framework 
for  purposes  of  bolting  down  to  angle  plates  fixed  to  the  shell. 
Cast-iron  coolers,  split  so  as  to  break  magnetic  circuits,  rest 
upon  the  roof  brickwork,  and  are  connected  to  water  circulation 
pipes  which  are  grouped  close  together  and  terminate  in  flexible 
hose  pipes. 

The  tilting  gear  is  of  the  screw  feed  type,  which  is  clearly 
shown  in  Fig.  108.  A  heavy  tilting  screw  is  connected  by  a 
pin  joint  to  the  steel  casting  which  separates  the  two  rockers, 


250  THE    ELECTRO-METALLURGY   OF    STEEL 

and  works  in  a  heavy  phosphor-bronze  nut,  journalled  in  a 
cast  steel  trunnion  box.  This  nut  is  bolted  to  a  large  bevel 
wheel,  which  is  rotated  by  a  small  bevel  pinion  driven  by  motor 
through  reduction  gearing.  The  end  thrust  on  the  nut  is  taken 
on  a  heavy  ball  race,  another  ball  race  being  provided  on  the 
under  side  to  prevent  any  possible  axial  movement  in  an  upward 
direction.  A  telescopic  dust  guard  covers  the  screw.  This 
type  of  tilting  gear  should  be  cleaned,  oiled,  and  greased  at 
regular  intervals  to  prevent  excessive  wear  of  the  screw  threads. 
This  method  of  tilting  is  widely  adopted  for  other  types  of 
furnaces  in  Great  Britain. 

The  l|-ton  furnace,  shown  in  Fig.  109,  embodies  the  same 
general  principles  of  construction  as  above  described,  but  a 
special  feature  is  introduced  by  the  provision  of  two  swivel  arms 
for  carrying  the  ladle.  By  this  means  an  overhead  casting  crane 
in  the  furnace  bay  can  be  dispensed  with.  This  arrangement  is 
used  in  conjunction  with  a  special  bogie,  which  serves  two  pur- 
poses, according  to  whether  ingots  or  castings  are  being  made  :— 

(a)  for  the  purpose  of  teeming  ingots,  the  transfer  bogie  is 
mounted  on  rails,  which  are  supported  above  and  on  either  side 
of  the  ingot  pit.     The  ingot  moulds  are  set  carefully  in  line  with 
the  teeming  nozzle,  and  can  be  filled  successively  by  carefully 
controlling  the    travelling   movement   of   the   bogie,   which  is 
effected  by  a  spur  wheel  and  pinion  drive  ; 

(b)  for  foundry  purposes  the  bogie  is  merely  used  for  trans- 
ferring the  ladle  from  the  furnace  to  the  casting  bay,  where  the 
ladle  is  then  handled  by  a  casting  crane. 

The  method  of  using  this  ladle  carriage  for  the  transfer  of 
the  ladle  to  and  from  the  furnace  is  as  follows : — 

The  removable  bogie  rails  which  span  the  ladle  pit  are  placed 
in  position  in  readiness  for  pouring.  The  ladle,  which  is  pro- 
vided with  extended  double  trunnions,  is  slung  on  the  bogie 
and  run  up  to  the  position  shown  in  the  figure.  The  ladle  arms 
are  swung  inwards,  and  the  furnace  is  slightly  tilted  backwards, 
so  that  the  arms  lift  the  ladle  clear  of  the  bogie  trunnion  bear- 
ings. The  bogie  is  then  moved  backwards  clear  of  the  ladle 
pit,  and  the  detachable  pieces  of  bogie  rails  removed.  The 
furnace  can  then  be  tilted  forwards  and  poured,  and  again 
brought  back  to  its  original  position.  The  rails  are  replaced, 


MODERN   TYPES   OF   ELECTRIC   STEEL   FURNACES 


251 


, 


252 


THE    ELECTRO-METALLURGY   OF   STEEL 


the  bogie  again  brought  up,  and  the  previous  cycle  of  operations 
reversed.  The  bogie  then  carries  the  ladle  of  steel  and  is  free 
to  serve  either  of  the  above  purposes. 

Furnace  Lining. — The  furnace  is  equally  suited  for  either 
the  acid  or  basic  process,  as  the  hearth  is  not  called  upon -to 
carry  any  current.  The  methods  adopted  for  lining  with  either 
acid  or  basic  material  are  those  which  are  fully  described  in 
Chapter  XIV. 

Electrodes. — Both  amorphous  and  graphite  electrodes  are 
used,  the  latter  being  the  more  suitable  for  furnaces  of  2  ton 
capacity  and  under.  Amorphous  electrodes  are,  at  present, 
almost  exclusively  used  for  the  larger  furnaces,  the  diameters 
varying  from  14  to  20  inches.  Economisers  of  special  design 
are  described  in  Chapter  XV. 

Qirod  Furnace. — The  original  design  of  the  Girod  furnace 
was  characterised  by  metallic  electrodes,  which  penetrated  the 


FIG.  110. 

hearth  and  electrically  connected  the  furnace  charge  to  one  of 
the  line  conductors. 

Electrical  Features. — The  furnaces  are  designed  to  operate 
on  either  single  or  three-phase  low  tension  systems.  The  dia- 
grams in  Fig.  110  show  three  methods  that  have  bean  used  for 
supplying  single-phase  current  to  the  furnace  electrodes.  In 
the  first  two  instances  the  bottom  electrodes  were  insulated 
from  the  furnace  body,  whereas  according  to  the  latest  method 
they  are  electrically  connected  to  the  steel  shell  plates,  to  which 
is  directly  attached  one  set  of  the  conductors.  The  small 
arrows  indicate  the  direction  in  which  the  arc  is  deflected  in 
each  case,  the  deflection  being  due  to  magnetic  fields  set  up  in 
the  steel  shell  by  the  heavy  alternating  current  traversing  the 
bus  bars  in  close  proximity  to  it.  At  the  Gutehoffnungshutte 
the  local  destruction  of  the  furnace  walls  was  so  considerable 
that  it  was  eventually  found  cheaper  to  adopt  the  third  and 


MODEEN  TYPES  OF  ELECTBIC  STEEL  FUENACES 


253 


more  symmetrical  method  of  bus  bar  arrangement,  which  en- 
tailed the  use  of  extra  copper  and  resulted  in  a  rather  lower 
power  factor. 

The  actual  arrangement  of  the  conductor  bars  is  more  clearly 
shown  in  Fig.  ill.  The  bars  are  brought  interleaved  from  the 
generator  to  a  point  "US"  underneath  the  furnace,  whence 
they  are  split  into  two  separate  sets ;  each  set  consists  of  con- 
ductors of  opposite  polarity  similarly  interleaved,  which  are 


FIG.  111. 

carried  up  to  a  point  level  with  the  rolling  axis  of  the  furnace. 
Here,  either  cables  or  flexible  strips  are  used  for  making  the 
short  connections  both  to  the  electrode  bus  bars  and  to  the 
furnace  body.  The  steel  shell,  below  the  point  where  the 
flexible  connections  are  made,  is  under  the  influence  of  alter- 
nating currents  of  similar  magnitude  and  opposite  phase,  so 
that  the  magnetic  effects  are  neutralised.  In  this  way,  only 
very  slight,  rotating  magnetic  fields  are  set  up  around  the 


254  THE    ELECTRO-METALLURGY   OF   STEEL 

carbon  electrode,  which  results  in  more  uniform  heating  of  the 
furnace  charge  and  lining. 

The  system  of  connections  used  for  the  three-phase  furnace 
is  exceedingly  simple.  The  three  low  tension  phases  are  star 
connected,  the  outer  terminal  of  each  phase  being  connected  to 
an  upper  adjustable  carbon  electrode,  while  the  star  point  is 
connected  to  a  series  of  metallic  pole  pieces  fixed  to  the  bottom 
plate  and  embedded  in  a  conductive  hearth.  With  this  arrange- 
ment, when  the  load  is  equally  balanced  between  the  three  arc 
circuits,  no  current  will  flow  through  the  bottom  electrodes  and 
the  return  conductor ;  the  phase  or  open  circuit  arc  voltage  is 
generally  about  65  volts. 

Furnace  Design. — The  3-ton  single-phase  furnace,  as  used  at 
the  Gutehoffnungshiitte,  is  shown  in  section  in  Fig.  Ill,  the  body 
in  this  case  being  square.  The  six  bottom  electrodes  are  electri- 
cally connected  by  means  of  a  copper  ring  and  plate  with  each 
other  and  with  the  furnace  body.  The  furnace  shell  is  mounted 
on  rockers  resting  upon  roller  mountings. 

The  ratio  of  the  cross-section  of  the  steel  electrodes  to  the 
rest  of  the  bottom  area  is  as  1  to  16.  These  electrodes  are  4 
inches  in  diameter  at  their  upper,  and  6J  inches  at  their  lower 
extremities,  which  project  about  8  inches  below  the  furnace 
bottom.  The  projecting  portion  has  a  cylindrical  cavity  5J 
inches  long,  through  which  water  circulates  to  prevent  excessive 
melting  of  the  electrode  at  its  upper  exposed  end. 

1 A  10-ton  furnace,  for  melting  and  refining  cold  scrap 
charges,  was  put  into  commission  at  the  works  of  the  Bethlehem 
Steel  Company,  U.S.A.,  in  1916.  Furnaces  of  this  capacity  are 
designed  for  three-phase  operation  and  are  constructed  circular 
in  shape.  The  furnace  shell  is  5  feet  in  depth  and  15  feet  in 
diameter.  A  large,  single  charging  door,,  sliding  in  a  water- 
cooled  frame,  is  provided  on  one  side  of  the  furnace  and  im- 
mediately opposite  the  pouring  spout.  The  furnace  can  be 
tilted  either  forwards  or  backwards,  so  that  slag  can  be  poured 
off  through  a  notch  in  the  charging  door  sill.  Fourteen  soft 
steel  electrodes,  about  3-£  inches  in  diameter,  are  electrically 
connected  to  the  furnace  bottom  plate,  the  lower  ends  being 

1  American  Electro  Chemical  Society. 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     255 

water-cooled  as  usual.  The  electrode  carriers  are  mounted  in 
structural  columns,  which  are  fixed  on  opposite  sides  of  the  furnace 
and  are  raised  and  lowered  by  a  screw  and  nut  feed.  Special 
care  is  taken  to  insulate  the  electrode  bus  bars,  and  to  prevent 
induced  currents  in  the  shell  and  roof  frame.  The  furnace  is 
supplied  with  power  from  a  group  of  three  single-phase  trans- 
formers having  a  total  capacity  of  700  K.V.A.,  each  of  which  is 
protected  by  a  reactance  coil  of  106  K.V.A.  capacity. 

Furnace  Lining. — The  single-phase  furnace  referred  to  above 
was  originally  lined  with  magnesite,  which  was  later  given  up  in 
favour  of  dolomite  for  both  the  hearth  and  wall  construction. 
When  the  furnace  was  used  for  liquid  refining,  the  hearth  and 
walls  would  generally  last  about  120  heats,  the  hearth,  originally 
18  inches  thick,  dropping  about  two  inches  during  this  period. 
The  roof  was  lined  with  silica  bricks  springing  from  magnesite 
skewbacks  to  admit  of  its  easy  detachment  from  the  dolomite 
walls,  which  was  generally  found  necessary  after  60  or  70  heats. 
The  heat  loss,  due  to  water  cooling  the  bottom  electrodes,  was 
carefully  determined  by  measuring  the  quantity  of  water  flowing 
and  its  temperature  before  and  after  passage  through  them. 
The  loss  was  equivalent  to  an  energy  consumption  of  only  2*9 
K.W.  hours  per  ton  of  steel,  and  is  small  compared  to  the  loss  of 
10*5  K.W.  hours  per  ton  of  steel  measured  at  the  electrode 
cooling  jacket.  The  amount  of  water  required  for  cooling  the 
bottom  electrodes  was  only  "20  cubic  meters  per  ton  of  steel  as 
compared  with  "65  cubic  meters  required  for  the  top  electrode 
cooling  ring.  With  the  proper  degree  of  water-cooling  the 
steel  electrodes  should  only  melt  to  a  depth  of  about  one  or  two 
inches  below  the  hearth  level,  so  that  no  serious  erosion  of  the 
dolomite  results. 

In  the  larger  three-phase  basic  furnace  the  wall  lining  is 
built  of  magnesite  brick  up  to  the  roof,  from  which  it  is 
separated  by  asbestos  plates ;  in  other  respects  the  lining  does 
not  differ  from  standard  practice. 

Electro -Metals  Furnace. — The  original  feature  of  the  Electro- 
Metals  furnace  was  the  application  of  a  three-wire  two-phase 
system  of  low  tension  connections.  This  design  enables  either 
two  or  three-phase  high  tension  current  supplies  to  be  used 
without  the  aid  of  motor  generators,  which  were  formerly 


256  THE   ELECTEO-METALLUEGY   OF    STEEL 

necessary  for  single-phase  furnace  operation.  A  conductive 
hearth  is  still  an  essential  characteristic  of  this  furnace. 

Electrical  Features. — The  method  by  which  two-phase 
current  is  supplied  to  this  furnace  has  been  fully  described  in 
Chapter  III.,  and  does  not  require  further  explanation.  A 
diagram  of  the  entire  electrical  equipment  is  shown  in  Fig.  112. 
Here,  the  two-phase  low  tension  current  is  transformed  down 
from  a  three-phase  high  tension  supply  by  Scott-connected 
transformers.  Tappings  are  taken  out  from  the  primary  windings 
to  give  either  90,  80,  or  70  volts  across  each  of  the  low  tension 
phases  on  open  circuit,  and  the  various  connections  can  be 
made  by  two  selector  switches  A  and  B,  which  are  interlocked 
with  one  another,  and  with  the  main  oil  switch  O.S.A.  In  the 
7^-ton  furnace  rather  higher  open  circuit  voltages  are  used, 
namely,  100,  85,  and  75.  The  neutral  conductor  cables  are  con- 
nected to  copper  bars  placed  side  by  side  on  a  course  of  bricks 
laid  on  the  bottom  plate ;  details  of  this  method  of  conveying 
current  to  the  hearth  are  given  in  Chapter  XIV.  (Fig. 
126).  A  furnace  operating  on  a  four-phase  low  tension  system 
has  been  recently  designed  for  large  capacities.  The  special 
method  of  transformer  grouping  adopted  for  supplying  such 
four-phase  low  tension  current  from  a  three-phase  supply  has 
also  been  dealt  with  in  Chapter  III. 

The  large  power  inputs  required  for  furnaces  exceeding  10 
tons  capacity  cannot  be  satisfactorily  carried  by  only  two  elec- 
trodes, to  which  number  the  two-phase  pattern  is  limited,  and 
it  is  chiefly  for  this  reason  that  four-phase  current  requiring  four 
upper  electrodes  is  employed.  This  system  requires  five  separate 
sets  of  conductors  from  the  transformer  group,  four  being  con- 
nected to  the  upper  electrode  bus  bars,  and  the  other  to  copper 
bars  imbedded  in  the  furnace  hearth  in  a  manner  similar  to  the 
two-phase  furnace.  The  hearth  is  only  called  upon  to  carry 
rather  more  than  the  current  flowing  through  any  one  elec- 
trode when  all  are  equally  balanced,  and  the  current  density  is 
one-third  of  that  of  the  two-phase  three  wire  type.  The  load  is 
controlled  and  balanced  by  ammeters,  which  register  the  current 
in  each  arc  circuit.  Voltage  variation  is  also  provided  for  by 
tappings  taken  from  the  high  tension  transformer  windings  in 
the  usual  way. 


p 

<Q  ' 


ELECTRK:  CONNECTIONS 

TO  ELECTRdOE 


CENTRE  LINE 
OF  ROCKER  FOR  MAXIMUM 
FORWARD  TILT 


[To  face  p.  257. 


MODEEN  TYPES  OF  ELECTRIC  STEEL  FURNACES     257 

Furnace  Design. — The  furnace  body  of  the  two-phase  type 
is  built  in  the  form  of  a  rectangular  tank,  which  is  supported  on 
rockers  and  carries  the  necessary  structure  for  guiding  the  elec- 
trode carriages.  The  construction  of  the  7|-ton  furnace,  shown 
in  Fig.  113,  is  generally  similar  to  that  of  the  smaller  capacities. 
Each  of  the  columns,  which  serve  as  guides  for  the  electrode 
carriages,  consists  of  a  pair  of  channels  arranged  so  as  to  leave 
narrow  openings  between  their  inwardly  facing  flanges.  The 
electrode  carriages  encircle  these  guide  columns,  and  each  one 
consists  of  two  steel  plates  rigidly  bolted  together  at  four  corners 
as  shown ;  guide  rollers  are  mounted  on  two  of  these  bolts  in 
such  a  manner  that  the  weight  of  the  arm  always  causes  it  to 
grip  the  column  and  allow  vertical  adjustment  without  lateral 
or  other  movement.  A  long  vertical  screw  is  mounted  centrally 
within  each  column,  and  passes  through  a  bronze  nut  fastened 
to  the  electrode  carriage.  Rotation  of  this  screw,  which  is 
effected  by  hand  or  rnotor  driven  gearing,  raises  or  lowers  the 
electrode.  A  protective  device  is  introduced  to  prevent  damage 
to  the  raising  gear  in  the  event  of  an  electrode  being  forcibly 
driven  against  a  resisting  obstacle.  The  electrode  arm 
terminates  in  a  collar,  which  supports  the  electrode  clamping 
device  and  carries  the  weight  of  the  electrode  so  held. 
Horizontal  conductor  bars  are  fastened  to  each  electrode  arm 
by  insulated  clamps,  and  carry  suitably  bent  copper  strips  at 
one  end  and  cable  sockets  at  the  other.  The  electrode  clamp 
encircles  the  copper  conducting  strips,  which  are  bent  to  the  ap- 
proximate diameter  of  the  electrode,  and  consists  of  a  small  num- 
ber of  steel  links  hinged  together  and  capable  of  being  tightened 
up  by  means  of  a  right  and  left  hand  screw.  The  connections 
from  the  horizontal  conductors  to  the  transformer  terminals  are 
made  with  flexible  cables.  The  tilting  gear  and  electrode 
regulating  mechanism  are  shown  in  the  back  elevation  drawing 
of  Fig.  113.  The  rectangular  roof  frame  is  of  very  simple  con- 
struction, being  arched  about  the  longitudinal  axis  only. 

The  four-phase  furnace  is  illustrated  in  Fig.  114,  which 
shows  the  disposition  of  the  four  electrode  columns  and  motor 
platforms.  The  principle  adopted  for  guiding  the  electrode 
carriages  is  very  similar  to  that  already  described.  The  furnace 
shell  is  circular,  but  the  relative  position  of  the  charging  and 


258 


THE    ELECTROMETALLURGY   OF    STEEL 


slagging  doors  is  unaltered.     The  bottom  hearth  connection  is 
similar  to  that  of  the  two-phase  pattern. 

Furnace  Lining. — The   Electro-metals   furnace,    being   de- 
pendent upon  a   low  hearth   resistance,  is  always   lined   with 


Fm.  114. 


basic  material  from  the  bottom  to  above  the  slag  line.  The 
method  of  lining  furnaces  of  this  type  is  fully  described 
in  Chapter  XIV.  A  chrome  brick  parting  between  the 
magnesite  bricks  and  silica  walls  is  sometimes  used.  Furnaces 
from  2  to  7  ton  capacity  are  provided  with  thick  linings,  the 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     259 

walls  being  14  inches  and  the  hearth  20  inches  thick ;  these 
dimensions  are  also  retained  for  the  largest  size  yet  designed. 

Electrodes. — Both  amorphous  and  graphite  electrodes  may 
be  used,  but  the  latter  variety  is  generally  more  convenient 
owing  to  the  high  input  which  has  to  be  carried  by  only  two 
electrodes  in  the  two-phase  type.  Apart  also  from  the  necessity 
of  limiting  the  diameter  of  the  electrode  to  conform  to  the 
internal  furnace  dimensions,  there  are,  no  doubt,  other  factors 
which  have  led  to  the  general  adoption  of  graphite  electrodes. 
Economisers  of  special  design  are  also  used  (see  Chapter  XV.). 

Furnace  Operation. — Melting  operations  can  only  be  con- 
ducted under  satisfactory  electrical  conditions,  when  the  con- 
ductive hearth  is  capable  of  carrying  practically  the  full  return 
current  of  the  two  individual  phase  circuits.  This  is  only 
possible  when  the  refractory  hearth  is  at  a  high  temperature, 
and  its  conductivity  thereby  materially  improved. 

The  method  of  heating  used  for  baking  the  hearth  of  a 
newly  lined  furnace  is  fully  described  in  Chapter  XIV.,  and 
may  be  adopted  for  heating  up  an  old  lining  after  a  prolonged 
stoppage.  Gas  or  oil  heating  may  be  equally  well  employed 
when  circumstances  permit. 

When  cold  charges  are  being  melted,  the  scrap  is  charged  in 
the  ordinary  way  mixed  with  lime  and  ore,  or  mill  scale.  Arcs 
are  struck  by  lowering  the  electrodes,  and,  if  the  bottom  is 
sufficiently  hot,  the  current  flowing  through  the  neutral  return 
conductor,  as  shown  by  an  ammeter,  will  be  practically  1*4  times 
the  current  flowing  through  either  of  the  electrode  circuits 
when  the  latter  are  balanced.  It  will  usually  be  found  that 
just  at  first  the  hearth  is  not  capable  of  carrying  this  proportion 
of  current,  and  some  operators  then  prefer  to  work  at  a  slightly 
reduced  load  until  the  proper  electrical  conditions  are  fulfilled. 
When  successive  heats  are  being  melted  the  hearth  will  generally 
carry  the  full  return  current  after  a  few  minutes,  and  it  is  only 
after  stoppages  that  care  requires  to  be  exercised.  Even  though 
the  current  flowing  through  each  electrode  circuit  may  be  the 
same,  the  primary  supply  circuit  will  not  properly  balance  unless 
the  neutral  conductor  is  carrying  the  correct  proportion  of 
current  ;  by  reducing  the  load,  however,  to  enable  the  hearth 
to  satisfy  this  condition,  correct  balance  on  the  power  supply 


260  THE    ELECTKO-METALLUKGY   O'F    STEEL 

system  is  obtained.  The  indicating  instruments,  consisting  of 
three  ammeters,  a  voltmeter,  and  an  indicating  wattmeter,  are 
mounted  on  a  panel  rather  behind  and  to  one  side  of  the  furnace. 
When  the  transformer  substation  is  situated  above  floor  level 
and  close  behind  the  furnace,  the  instrument  panel  is  generally 
mounted  on  the  dividing  wall,  and  then  also  carries  the  selector 
and  oil  switches.  Two  tramway  type  controllers  for  operating 
the  electrode  motors  are  mounted  at  the  foot  of  this  panel,  and 
can  be  conveniently  handled  by  a  furnaceman  seated  in  front  of 
them  and  facing  the  ammeters. 

Automatic  load  regulation  is  generally  adopted,  each  elec- 
trode being  controlled  by  a  regulator  actuated  by  a  current 
transformer  in  each  of  the  separate  electrode  circuits. 

Stobie  Furnace. — The  original  Stobie  furnace  conforms  in 
general  principles  to  the  Electro-metals  design.  In  place,  how- 
ever, of  the  three- wire  two-phase  system  of  connections  adopted 
in  the  latter,  Btobie  keeps  the  two  low  tension  circuits  distinct, 
thereby  introducing  a  fourth  conductor.  The  use  of  a  conductive 
hearth  and  bottom  electrodes  has  since  been  abandoned  for 
furnaces  of  6  ton  capacity  and  over,  the  power  being  introduced 
by  four  upper  electrodes,  connected  to  a  three-phase,  and  more 
recently  to  a  two-phase  system. 

Electrical  Features. — The  arrangement  of  the  low  tension 
circuits  of  both  the  above-mentioned  types  has  been  explained  in 
Chapter  III,  (Figs.  45,  49).  In  the  four  top  electrode  two-phase 
system,  there  are  two  arcs  in  series  in  each  phase,  so  that  the 
method  of  automatic  regulation  has  to  be  based  upon  both  voltage 
and  current  control.  The  reactance  of  the  transformers  and  con- 
ductors is  such  that  the  power  factor  varies  from  '84  to  '93  at 
different  periods  of  melting  and  refining  a  charge  of  scrap.  The 
average  power  factor  is  stated  to  be  '88  in  the  case  of  the  large 
furnaces  operating  with  four  top  electrodes. 

Furnace  Design. — The  general  design  of  the  small  two-phase 
bottom  electrode  furnace  is  indicated  in  Fig.  115,  which  does  not, 
however,  show  the  cable  connections.  The  body  is  here  shown 
mounted  on  trunnions,  but  is  also  constructed  to  be  tilted  on 
rollers  by  a  motor-driven  winch  and  wire  cables.  There  is  only 
one  charging  door,  which  is  opposite  the  pouring  spout.  The 
electrode  raising  gear  is  a  distinct  departure  from  the  more 


MODERN   TYPES   OF  ELECTRIC   STEEL   FURNACES  261 


262  THE   ELECTBO-METALLtJRGY.    OF   STEEL 

usual  types,  which  embody  guide  columns,  electrode  carriages 
and  arms  ;  in  this  case  the  electrodes  are  suspended  from  ropes, 
which  are  wound  on  to  light  winches  mounted  on  small  extended 
platforms,  as  shown.  The  point  of  suspension  is  considerably 
elevated  to  allow  the  necessary  range  of  electrode  travel  above 
the  economisers,  which  are  about  2  feet  high.  The  roof  is 
arched  about  the  shorter  axis  of  the  body,  which  allows  more 
headroom  for  the  door  opening. 

The  larger  furnace  with  four  top  electrodes  is  octagonal  in 
shape,  and  is  provided  with  three  charging  doors  and  a  tapping 
spout.  The  electrodes  are  grouped  in  the  form  of  a  square,  and 
are  so  arranged  that  each  door  opening  faces  the  gap  between 
any  pair  of  adjacent  electrodes  ;  this  facilitates  charging  opera- 
tions and  general  manipulation. 

The  same  method  of  rope  suspension  is  adopted  for  the 
electrode  regulating  gear,  which,  being  symmetrically  situated 
above  the  furnace  shell,  allows  the  above-mentioned  arrange- 
ment of  door  openings  to  be  adopted.  Electrode  economisers 
as  described  in  Chapter  XV.  are  always  used,  and  are  generally 
regarded  as  the  most  characteristic  feature  of  the  Stobie  fur- 
nace. 

Furnace  Lining. — Furnaces  of  5  tons  capacity  or  less 
operate  on  the  conductive  hearth  principle,  and  are  consequently 
only  suitable  for  basic  linings.  It  Jhas  been  explained  that 
the  two-phase  low  tension  circuits  are  kept  distinct  as  far 
as  possible,  and  to  effect  this  purpose  a  bottom  electrode 
is  embedded  at  each  end  of  the  dolomite  hearth.  The 
large  octagonal  furnaces  with  four  upper  electrodes  may  be 
lined  with  either  acid  or  basic  materials,  since  the  hearth  is  in- 
dependent of  all  electrical  circuits  and  does  not  carry  current. 

Electrodes. — Stobie  furnaces  are  designed  for  use  with 
graphite  electrodes,  which  considerably  simplify  the  problem  of 
gas  sealing  at  the  points  where  they  enter  the  roof.  It  is  also 
pointed  out  by  Stobie  that  the  electrical  resistance  due  to  skin 
effect  increases  with  the  diameter  of  the  electrode,  and  that, 
although  the  relative  resistance  of  amorphous  carbon  and 
graphite  is  as  4  to  1,  the  cross  section  of  the  amorphous  carbon 
electrodes  must  bear  a  still  greater  ratio  for  similar  carrying 
capacities.  This  theory  is  advanced  as  a  reason  for  using 


MODERN   TYPES   OF  ELECTHIC   STEEL   FtTBNACES  263 

graphite  electrodes,  but  owing  to  the  comparatively  high  resist- 
ance of  both  varieties  the  skin  effect  is  not  very  pronounced. 

Snyder  Furnace. — The  Snyder  steel  furnace,  as  first  intro- 
duced, embodied  certain  electrical  and  mechanical  features, 
which  constituted  a  distinct  departure  from  other  then  existing 
types.  A  high  arc  voltage  and  considerable  circuit  reactance 
were  utilised  to  enable  a  steady  load,  and  therefore  a  high 
operating  load  factor,  to  be  maintained.  The  shape  of  the 
furnace  body  was  designed  to  approximate  as  far  as  possible  to 
a  sphere,  so  as  to  reduce  the  radiation  surface  to  a  minimum 
for  a  given  hearth  capacity.  Thick  linings  and  close  fitting 
charging  doors  of  a  special  type  were  also  used  to  obtain  a  high 
thermal  efficiency.  In  the  modern  pattern  even  the  side 
door  openings  are  dispensed  with,  provision  being  made  for 
charging  scrap  directly  into  the  furnace  body  by  introducing 
special  means  for  lifting  the  roof  after  each  heat. 

Electrical  Features. — Synder  furnaces  were  originally  de- 
signed for  single-phase  current,  but  are  now  also  constructed  for 
three-phase  operation.  The  arrangement  of  the  single-phase 
circuit  is  diagrammatically  shown  in  Fig.  30.  One  terminal  of 
a  single-phase  transformer  or  alternator  is  connected  to  an  upper 
adjustable  electrode,  the  other,  which  is  earthed,  being  connected 
to  a  metallic  electrode  which  penetrates  the  hearth.  A  choking 
or  reactance  coil  is  also  incorporated  in  the  circuit,  and  is  so 
designed  that,  when  the  normal  full  load  current  is  flowing,  the 
reactive  voltage  is  equal  to  the  voltage  drop  due  to  the  non- 
inductive  resistance  of  the  circuit.  Under  these  circumstances 
the  voltage  between  the  top  and  bottom  electrode  will  be  re- 
duced to  '71  times  the  voltage  across  the  transformer  terminals, 
and,  as  has  already  been  pointed  out  in  Chapter  IV.,  it  is  im- 
possible for  the  current  to  exceed  1'41  times  the  normal  full 
load  value,  even  on  dead  short  circuit.  If  the  current  flowing 
in  a  circuit  containing  such  a  reactance  coil  is  gradually  in- 
creased from  zero,  it  will  be  found  that 'the  voltage  across  the 
practically  non-inductive  portion,  which  in  this  case  lies  between 
the  electrodes,  will  progressively  fall,  and,  assuming  the  power 
factor  of  the  circuit  through  the  furnace  to  be  known,  it  will 
then  be  possible  to  calculate  the  kilowatts  delivered  to  the  fur- 
nace itself  for  various  current  values.  The  curve  A  in  Fig.  116 


'264 


THE   ELECTRO-METALLUBGY   OF   STEEL 


KlUOWATTS. 

O  O 

O  O 

o  m 


i      8 


oo     ooooo 
ojoo^vOio^o 

—       —         —         —        —         —        — 

•SJ-HO/\ 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     265 

has  been  plotted  from  actually  measured  kilowatt  inputs  corre- 
sponding to  current  values,  ranging  from  zero  to  the  maximum 
obtainable  on  short  circuit.  In  this  case  the  external  reactance 
was  introduced  in  the  primary  circuit  of  the  furnace  transformer 
in  the  form  of  a  choking  coil,  which  was  provided  with  tappings, 
so  that  by  means  of  a  selector  switch  various  degrees  of  re- 
actance could  be  introduced.  The  curves'  B  and  C  show  the 
variation  of  power  input  for  different  current  values  after  further 
increasing  the  amount  of  reactance.  Curves  1,  2,  and  3  were 
plotted  from  simultaneous  readings  of  the  furnace  terminal 
voltage  and  current,  and  it  will  be  seen  that  the  product  of  two 
such  readings  gives  a  K.V.A.  value  about  equal  to  the  corre- 
sponding K.W.,  as  actually  indicated  by  a  wattmeter.  Further, 
the  K.V.A.  values  as  calculated,  or  the  K.W.  actually  measured, 
are  fairly  constant  within  a  certain  range  of  current  variation, 
and  it  is  this  special  characteristic  of  such  high  reactance  circuits 
that  renders  the  power  input  capable  of  self -adjustment  within 
certain  limits  of  current  variation.  If  during  the  melting 
operation  the  arc  length  is  once  adjusted  so  that  the  normal 
full  load  current  flows,  then,  notwithstanding  a  slight  change 
in  the  arc  resistance  due  to  alteration  of  the  relative  position  of 
the  electrode  and  the  charge,  the  power  input  will  remain 
practically  unaltered.  Partial  short  circuits  caused  by  scrap 
falling  against  the  electrode  will  be  accompanied  by  a  diminished 
power  input,  which  is  contrary  to  the  effect  produced  in  furnace 
circuits  provided  with  only  12  per  cent,  to  30  per  cent,  trans- 
former circuit  reactance.  A  typical  load  chart  is  illustrated  in 
Fig.  117,  and  shows  the  remarkable  steadiness  of  the  load,  and 
high  load  factor  rendered  possible  during  the  entire  melting  and 
refining  operation.  A  further  feature  of  the  electrical  design, 
conducive  to  steadiness  of  load,  is  the  high  arc  voltage,  which, 
at  the  maximum  working  load,  varies  according  to  the  reactance 
of  the  circuit.  In  the  case  of  the  installation  for  which  the 
curves  in  Fig.  116  were  plotted,  the  open  circuit  secondary 
voltage  across  the  furnace  terminals  was  220  volts  ;  this  voltage 
dropped  to  135,  122,  and  103  volts  at  maximum  load  according 
to  the  reactance  introduced  by  a  reactance  coil  provided  with 
three  interchangeable  tappings. 

On  account  of  the  high  reactance  used  to  stabilise  the  load, 


266 


THE   ELECTKO-METALLUKGY   OF    STEEL 


the  power  factor  is  considerably  lowe'red  and  must  be  •?,  or 
less,  at  normal  full  load,  if  the  special  characteristics  of  load 
self-adjustment  are  to  be  fully  realised.  Automatic  regulation, 


I 


1 


although  by  no  means  essential,  is  sometimes  employed,  and  in 
that  case  relieves  the  furnace  operator  from  even  the  occasional 
attention  otherwise  required. 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     267 

Three-phase  current  has  also  been  used  for  operating  Snyder 
furnaces.  Two  line  conductors  connect  two  terminals  of  the 
three-phase  system  to  two  upper  carbon  electrodes,  a  third  con- 
ductor being  used  to  make  the  connection  between  a  bottom 
metallic  electrode  and  the  third  terminal.  With  a  simple  star 
or  delta  grouping  it  is  obvious  that  proper  distribution  of  the 
load  between  the  three-phases  is  impossible,  unless  the  resistance 
in  each  circuit  is  equal,  and  for  this  reason  some  form  of  com- 
pensating device  is  no  doubt  introduced.  In  the  Greaves- 
Etchells  furnace  a  similar  problem  is  encountered,  and  is 
overcome  by  adopting  some  special  form  of  transformer  group- 
ing, whereby  the  suitable  line  voltage  between  different  pairs  of 
the  line  conductors  is  obtained. 

Load  variation  over  wide  ranges  is  not  such  a  simple  problem 
as  in  the  case  of  furnaces  designed  with  only  limited  circuit 
reactance.  In  the  latter  case  the  circuit  resistance  may  be 
increased  by  simply  lengthening  the  arc,  which  causes  a  lower- 
ing of  the  current  without  any  material  rise  of  voltage,  and 
thus  reduces  the  load.  This  simple  expedient  does  not  suffice 
where  high  circuit  reactance  is  introduced,  since  on  lengthening 
the  arc  the  current  fall  is  accompanied,  up  to  a  certain  point, 
by  a  corresponding  voltage  rise.  Therefore,  in  order  to  effect 
any  considerable  reduction  of  load,  it  is  necessary  to  lengthen 
the  arc  to  such  an  extent  that  it  becomes  far  less  stable  and  has 
a  more  intense  heating  effect  on  the  furnace  lining.  According 
to  one  method  that  has  been  employed  for  effecting  variation 
of  load,  the  furnace  terminal  voltage  is  reduced  by  introducing 
extra  reactance  into  the  circuit,  and  the  three  curves  A,  B,  and  C 
in  Fig.  116  show  the  different  loads  at  which  it  is  possible  to 
work  for  the  particular  case  mentioned.  With  this  arrange- 
ment the  furnace  may  be  operated  under  proper  electrical 
conditions  at  either  420,  360,  or  260  K.W.,  but  a  still  lower 
load  can  only  be  obtained  by  lengthening  the  arc ;  other 
methods  of  voltage  variation  may  also  be  adopted  for  effecting 
load  adjustment. 

Furnace  Design. — From  a  purely  constructional  standpoint, 
there  are  two  distinct  designs  of  Snyder  furnaces.  In  the  more 
recent  enclosed  body  type  there  is  only  one  small  front  opening 
ui  the  furnace  lining  for  the  purpose  of  pouring,  inspection, 


268 


THE   ELECTRO-METALLURGY  OF   STEEL 


bath  manipulation  and  sampling;  also,  the  entire  charge  of 
scrap  can  be  introduced  into  the  well  of  the  furnace  from  above, 
owing  to  the  special  mechanical  means  provided  for  lifting  the 


FIG.  118. 


roof.  A  side  elevation  of  a  5-ton  furnace  constructed  on  this 
principle  is  shown  in  Fig.  118.  For  furnaces  of  i-ton  capacity, 
and  under,  the  same  roof-lifting  principle  is  adopted,  but  the 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES 


269 


entire  furnace  is  mounted  on  trunnions  and  designed  for  hand 
tilting. 

The  earlier  type,  in  which  two  charging  doors  are  provided, 
does  not  permit  of  trunnion  tilting.  The  doors  were  originally 
introduced  as  one  of  the  special  features  of  the  Snyder  design  ; 
they  are  constructed  in  the  form  of  thick,  slightly  conical  brick 
plugs,  held  in  a  strong  framework  which  fits  tightly  to  the 
furnace  shell.  The  door  frame  is  pivoted  about  a  vertical  axis 
between  two  short  horizontal  arms,  which  can  be  swung  out- 
wards from  the  furnace  body.  This  construction  is  not  suitable 
for  sampling  and  inspection  purposes,  as  the  plug  is  generally 
luted  at  its  inner  end  after  charg- 
ing to  effect  a  close  seal ;  at  the 
same  time,  it  does  not  admit  of 
being  partially  opened,  and  for 
this  reason  the  spout  opening  is 
alone  used  for  the  general  mani- 
pulation subsequent  to  charging. 

Furnace  Lining. — Snyder  fur- 
naces, owing  to  the  high  arc 
voltage  used,  are  generally  more 
suitable  for  acid  operation,  al- 
though not  precluded  from  the 
use  of  basic  linings. 

A  section  through  an  acid 
lined  furnace  is  shown  in  Fig. 
119 ;  the  working  hearth  consists  of  a  suitable  ganister  mixture, 
which  is  made  and  rammed  to  shape  according  to  the  method 
described  later  in  Chapter  XIV. 

When  a  basic  lining  is  required,  the  circular  wall  of  magnesite 
bricks  is  built  up  to  a  level  slightly  above  the  slag  line,  from 
which  point  the  wall  may  be  constructed  by  ramming  "  black 
basic"  between  the  shell  plates  and  a  strong  template;  the 
working  hearth  is  likewise  rammed  in  the  usual  way,  using 
prepared  dolomite  in  place  of  ganister.  In  either  case  silica 
bricks  are  used  for  the  roof  construction. 

Furnace  Operation. — A  method  which  may  be  employed  for 
heating  a  newly  lined  furnace  is  described  in  Chapter  XIV., 
although,  more  especially  in  the  case  of  acid  linings,  such 


FIG.' 119. 


270  THE   ELECTRO-METALLURGY   OF   STEEL 

treatment  preliminary  to  melting  scrap  is  frequently  dispensed 
with.  It  is  customary  to  introduce  the  entire  charge  of  scrap, 
either  by  lifting  the  roof  or  through  the  charging  doors,  before 
starting  to  melt,  so  that  during  the  entire  process  of  steel-making 
the  furnace  remains  closed,  with  the  exception  of  the  one  small 
opening  above  the  pouring  spout.  After  striking  the  arc  the 
electrode  is  adjusted  until  either  the  correct  arc  voltage  or  cur- 
rent is  obtained,  either  of  which  figures  is  chosen  to  correspond 
to  the  crest  of  the  power  current  curves  A,  B,  and  C  of  Fig.  116. 
Melting  will  proceed  up  to  a  point  without  materially  affecting 
the  load,  and  only  occasional  adjustment  of  the  electrode  will 
be  required  before  the  charge  is  completely  melted. 

Special  precautions  are  taken  after  pouring  each  heat  to 
ensure  a  perfect  circuit  from  the  bottom  electrode  through  the 
scrap  to  the  upper  electrode  for  the  next  heat.  Any  slag  must 
be  carefully  scraped  away  from  the  small  pool  of  steel,  which 
marks  the  upper  end  of  the  metallic  electrode,  and  then  a  cast 
steel  spigot  about  9  inches  long  is  stood  upright  in  the  pool  of 
steel  until  the  latter  solidifies.  The  whole  operation  must  be 
done  most  expeditiously  before  the  pool  sets. 

The  wear  on  the  roof  and  lining  is  naturally  severe  owing 
to  the  exposed  nature  of  the  high  voltage  arc,  but  is  not  so  pro- 
nounced with  acid  linings  owing  to  the  shorter  duration  of  each 
heat  and  the  absence  of  basic  dust-laden  gases. 

The  excellent  operating  load  factor  made  possible  by  intro- 
ducing a  high  circuit  reactance  is  a  feature  that  naturally  con- 
duces to  the  maximum  possible  output  of  steel  for  a  given 
available  power  input.  Thirty  heats  have  been  regularly  ob- 
tained during  five  days  constant  operation  with  a  2-ton  furnace 
supplied  with  a  maximum  power  of  420  K.W.  ;  in  this  case  the 
furnace  was  acid  lined  and  used  for  making  mild  steel  castings 
from  good  turnings  scrap,  while  the  power  consumption  averaged 
about  710  units  per  ton  of  steel  in  the  ladle. 

Greaves- Etchells  Furnace. — Electrical  Features. — It  has 
been  explained  in  Chapter  III.  that  the  characteristic  feature  of 
the  electrical  design  lies  in  a  special  application  of  three-phase 
low  tension  current,  whereby  two-phases  are  connected  to  two 
upper  electrodes  and  one-phase  to  a  conductive  hearth.  If  the 
three-phase  system  comprised  three  similar  generating  circuits, 


MODERN   TYPES   OF   ELECTRIC   STEEL   FURNACES  271 

either  simple  star  or  mesh  connected,  then  it  would  be  impos- 
sible to  obtain  a  balanced  load,  since  the  resistance  in  each  of 
the  line  circuits  would  not  be  equal.  The  means  adopted  for 
overcoming  this  difficulty  were  also  briefly  indicated,  and  need 
not  be  further  dwelt  on,  as  full  explanation  could  only  be  given 
by  introducing  a  complex  mathematical  treatment  of  the 
problems  involved.  The  current  flowing  through  either  electrode 
always  traverses  two  of  the  low  tension  transformer  windings 
in  series.  It  is  pointed  out  that,  under  such  conditions,  any 
sudden  current  overload  will  cause  a  momentary  lowering  of 
the  power  factor,  which  thus  acts  as  a  limiting  factor  to  load 
fluctuation.  Voltage  variation  is  generally  provided  for  in  the 
usual  way  by  transformer  tappings,  and  load  regulation  is 
effected  either  by  automatic  or  hand  control. 

The  special  three-phase  system,  embodying  two  top  electrodes 
and  a  hearth  connection,  is  adopted  when  the  load  capacity 
does  not  exceed  1300  K.V.A.  When  transformers  of  larger 
capacity  are  used,  the  above  system  is  duplicated,  and  four  top 
electrodes  and  one  bottom  connection  are  then  required.  In 
this  way  the  power  input  can  be  greatly  increasd  without  over- 
loading the  electrodes.  The  electrical  power  plant  consists  of 
two  distinct  transformer  groups,  each  group  being  star  connected 
on  the  secondary,  and  mesh  connected  on  the  primary  side,  the 
transformer  ratios  of  each  pair  of  windings  being  designed 
according  to  the  same  manner  adopted  for  the  smaller  furnaces. 
Two  such  transformer  groups  may  be  combined  in  three  ways, 
so  that  the  current  flowing  through  the  hearth  is  either  '7  or 
•35  times  the  sum  of  the  currents  in  the  four  electrodes,  or  is 
reduced  to  nil.  Without  entering  into  a  full  explanation  of  the 
methods  of  combining  the  two  transformer  groups,  it  is  sufficient 
to  state  that  the  two  secondary  windings  connected  to  the 
conductive  hearth  are  arranged  to  be  either  in  phase,  120°  out 
of  phase,  or  180°  out  of  phase  with  one  another  to  produce  re- 
spectively the  above  proportions  of  hearth  current.  In  Fig. 
120  is  shown  a  cross  section  through  a  12-ton  furnace,  to- 
gether with  a  diagram  of  the  furnace  connections  from  one 
group  of  transformers.  The  short  leg  of  the  other  secondary 
group  is  similarly  connected  to  the  conductive  hearth,  and  its 
phase  will  bear  any  one  of  the  above  given  relationships  to  the 


272 


THE   ELECTRO-METALLURGY   OF    STEEL 


corresponding  leg  of  the  former  group.     By  the   provision  of 
tappings  in  the  primary  transformer  windings  it  is  also  possible 


to  work  at  three  different  voltages,  one  being  used  for  melting, 
one  for  refining,  and  one  when  holding  the  metal  at  a  constant 
temperature. 


MODERN  TYPES  OF  ELECTJlIC   STEEL  FtJfcKACES  273 

The  conductive  hearth  is  specially  constructed  to  offer  re- 
sistance to  the  passage  of  current,  and  from  8  per  cent,  to  10 
per  cent,  of  the  total  energy  supplied  to  the  furnace  can  be  con- 
verted into  heat  in  this  way.  By  this  means  of  applying  heat 
to  the  bottom  of  the  bath,  it  is  claimed  that  convection  currents 
are  induced,  which  produce  circulation  of  the  metal.  The 
effective  result  of  such  convection  currents  will  obviously  depend 
upon  the  difference  in  temperature  between  the  bottom,  middle, 
and  upper  layers.  The  latter  is  under  the  direct  influence  of 
arc  heating  and  must  be  at  the  highest  temperature  ;  the 
middle  layers  derive  heat  by  conduction,  and  would  normally  be 
above  the  temperature  of  the  bottom  layer.  The  degree  of 
bottom  heating  must  then  be  sufficient  to  raise  the  temperature 
of  the  lowest  layer  not  only  up  to  that  of  the  supernatant  metal, 
but  actually  above  it,  before  any  circulating  convection  current 
is  possible. 

Furnace  Design. — For  the  smaller  two-electrode  furnaces  the 
body  is  rectangular  in  shape,  and  provided  with  a  curved  bottom 
on  which  it  rolls  about  its  shorter  axis.  By  this  arrangement 
the  pouring  spout  is  fixed  at  one  end  of  the  body  and  a  charging 
door  at  the  other.  A  further  door  opening  is  provided  in  the 
long  side  of  the  body  facing  the  electrode  raising  columns, 
which  are  mounted  side  by  side.  The  method  of  guiding  and 
moving  the  electrode  carriages  is  substantially  the  same  as  that 
adopted  for  the  Electro-Metals  furnace ;  the  current  is  also 
conveyed  to  the  electrode  in  a  similar  manner  by  bent  conductors 
and  a  clamping  device,  which,  however,  is  constructed  in  the 
form  of  a  flexible  link  belting,  instead  of  using  a  small  number 
of  large  links. 

The  large  four  top  electrode  furnace  is  circular,  the  electrode 
columns  being  equally  spaced  around  the  body.  Both  the  large 
and  small  furnaces  are  supported  on  two  pairs  of  rollers,  and 
are  tilted  by  a  tilting  screw,  fed  forwards  or  backwards  by  the 
usual  swinging  trunnion  block  and  nut  device,  as  shown  in 
Fig.  121.  The  rollers  upon  which  the  furnace  body  rests  are 
themselves  supported  on  roller  carriages,  to  which  they  impart 
a  travelling  movement  when  the  furnace  is  tilted.  The 
diameters  of  the  supporting  roller  spindles,  and  of  the  tyres 
of  the  roller  carriage  wheels  upon  which  they  rest,  are  so 

18 


274 


THE    ELECTROMETALLURGY   OF    STEEL 


proportioned  that  the  travelling  movement  retains  the  lip  of  the 
spout  in  the  same  vertical  plane  during  pouring. 

Furnace  Lining. — Greaves-Etchells  furnaces  have  been  in- 
variably lined  with  basic  material  to  obtain  the  required  degree 
of  hearth  conductivity.  The  hearth  is  not,  however,  homo- 
geneous, but  is  rammed  in  layers  consisting  of  a  basic  mixture 


FIG.  121. 

of  varying  composition,  so  as  to  obtain  as  far  as  possible  an 
increasingly  higher  electrical  resistance  towards  the  top.  In 
this  way  the  greater  part  of  the  heat  is  generated  by  resistance 
in  close  vicinity  to  the  molten  metal.  Experiments,  have 
recently  been  made  with  a  conductive  acid  lined  hearth  and 
have  given  promise  of  ultimate  satisfaction  ;  no  details  of  its 
construction  are  as  yet  available. 

There  are  no  special  features  in  the  method  of  lining  apart 


MODEKN  TYPES  OF  ELECTEIC  STEEL  FURNACES 


275 


from  the  construction  of  the  hearth,  which  is  at  least  20  inches 
thick. 

Electrodes. — Either  amorphous  or  graphite  electrodes  can 
be  used,  but  the  latter  variety  is  more  generally  favoured.  The 
same  considerations  which  govern  the  selection  of  either  variety 
for  the  Electro-metals  furnace  are  equally  applicable  in  this 
case.  A  special  gland  sealing  type  of  economiser  is  also  used. 

Booth- Hall  Furnace. — Special  Characteristics. — This  fur- 
nace is  designed  to  operate 
with  a  two-phase  current, 
which  is  generally  trans- 
formed from  a  three-phase 
high  tension  system. 

The  low  tension  power 
connections  are  so  arranged 
that  a  conductive  hearth,  in 
the  case  of  the  basic  lined 
furnace,  may  be  made  an 
integral  part  of  the  power 
circuits,  which  are  otherwise 
independent  of  the  furnace 
lining  and  convey  current  to 
the  charge  through  three 
carbon  electrodes. 

Electrical   Design.  —  The 

two  secondary  phase  circuits  FIG  122 

of  a  Scott-connected  trans- 
former group  are  independently  connected  to  the  furnace 
conductors.  The  outer  terminal  of  each  phase  (Fig.  122)  is 
connected  to  an  upper  vertical  electrode,  the  inner  terminals 
being  connected  to  conducting  grids  embedded  in  a  basic 
hearth.  It  will  be  noticed  that  the  grid  in  each  circuit  is 
placed  diagonally  opposite  to  its  corresponding  upper  electrode, 
instead  of  vertically  beneath  it.  This  is  done  with  the  object 
of  lengthening  the  path  of  current  through  the  furnace  charge. 
A  switch  is  provided  for  short-circuiting  the  two  grids,  so  that 
the  two  phases  may  be  connected  to  a  common  neutral  point. 
A  third  auxiliary  electrode  is  permanently  connected  to  one  of 
the  grids,  and  is  intended  to  carry  the  return  current  from  the 


276  THE    ELECTEO-METALLUEGY   OF    STEEL 

two  main  electrodes  when  the  bottom  is  non-conductive,  the 
grids  being  short-circuited  for  this  purpose.  With  this  system 
of  two-phase  connections  it  is  possible  to  utilise  the  full  power 
of  the  transformers  at  a  time  when  the  bottom  conductive  hearth 
is  cold  and  inoperative.  The  grids  are  not  used  in  the  hearth 
construction  of  acid  lined  furnaces  ;  the  two  main  electrodes 
are  then  connected  direct  to  the  outer  terminals  of  each  phase, 
and  the  auxiliary  electrode  connected  through  the  neutral  return 
to  the  common  neutral  point  of  the  two  phases. 

The  auxiliary  electrode,  when  in  operation,  either  rests  upon 
the  charge  of  scrap  or  almost  touches  the  slag  covering  the 
bath.  The 'main  phase  electrodes  strike  direct  arcs  on  to  the 
charge  and  are  quite  independent  of  one  another  in  their  opera- 
tion. A  fluctuation  of  current  in  one  phase  will  not,  therefore, 
affect  the  other.  This  somewhat  simplifies  automatic  regula- 
tion and  eliminates  the  tendency  to  "  hunt  ". 

The  voltage  across  each  phase  at  the  transformer  terminals 
on  open  circuit  varies  from  75  to  150  volts,  depending  upon 
the  conditions  of  the  power  service.  Where  a  low  power  factor 
is  permissible  it  may  be  an  advantage  to  transform  to  the 
higher  voltage,  which  will  be  reduced  to  a  desirable  figure  at 
the  furnace  terminals  by  interposing  powerful  reactance  coils 
in  each  low  phase  circuit.  Such  reactance  coils,  which  cause 
a  reduced  power  factor,  would,  of  course,  be  installed  solely  for 
the  purpose  of  steadying  the  load  and  thereby  improving  the 
load  factor.  The  usual  voltage  variation  is  obtained  by  means 
of  transformer  tappings  and  selector  switches. 

The  load  rating  of  the  Booth-Hall  furnaces  is  exceptionally 
high,  as  the  following  figures  show : — 

f  ton 300KV.A. 

H  tons 700  „ 

3  „.-.'..  .  .  .  1400  „ 

6  „ 2100  „ 

15     „ 4200       „ 

Furnace  Design. — The  latest  type  of  furnace  has  an  elongated 
body  with  semicylindrical  ends,  so  that  the  two  main  vertical 
electrodes  can  be  set  equidistant  from  the  end  walls  at  all  points  ; 
this  construction  is  followed  so  as  to  obtain  uniform  heating 


MODERN  TYPES  OF  ELECTRIC  STEEL  FURNACES     277 

and  wear  of  refractories.  The  furnace  body,  which  has  a  dished 
bottom  plate,  is  mounted  on  rockers  and  tilts  about  its  shorter 
axis.  The  raising  gear  and  gallows  arm  mountings  for  the 
two  main  electrodes  are  attached  to  one  side  of  the  furnace 
body,  the  auxiliary  electrode  being  carried  in  a  holder  mounted 
on  an  inclined  bracket  fixed  centrally  on  the  opposite  side.  The 
auxiliary  electrode  is  inclined  downwards  towards  the  furnace 
interior  at  an  angle  of  about  45°,  and  passes  through  the  furnace 
shell  plate  at  a  point  about  midway  between  the  top  of  the 
walls  and  the  slag  line.  The  opening  in  the  brickwork  through 
which  this  electrode  passes  encloses  a  water-cooled  guide  box, 
which,  in  the  6-ton  furnace,  is  1  foot  long  and  penetrates  to 
within  6  inches  of  the  inner  face  of  an  18  inch  lining.  The 
furnace  is  designed  to  tilt  either  forwards  or  backwards  from  a 
vertical  position,  which  enables  slag  to  be  skimmed  over  one 
door  sill,  and  steel  to  be  poured  over  a  spout  situated  at  the 
opposite  end ;  this  arrangement  is  used  both  in  the  new  and  in 
the  older  cylindrical  body  types. 

In  the  earlier  types,  except  for  the  smaller  sizes,  the  auxiliary 
electrode  was  mounted  in  the  same  way  as  the  main  electrodes, 
and  passed  through  the  roof  at  a  point  midway  between  the  centre 
line  of  the  furnace  and  either  one  of  the  ends.  The  main  electrodes 
are  always  symmetrically  placed  in  relation  to  the  furnace  in- 
terior, as  shown  in  Fig.  123.  The  transverse  section  shows  two 
sets  of  cast  steel  grids,  which  are  embedded  in  a  basic  hearth 
and  insulated  from  one  another ;  the  bus  bar  connections  are  also 
shown.  The  electrodes  are  gripped  by  three  equally  spaced 
contact  plates ;  one  plate  is  fixed  to  the  end  of  the  gallows  arm, 
the  others  being  pivoted  on  the  ends  of  horizontal  levers,  which 
have  their  hinged  fulcrums  a  few  inches  behind  the  rigid  contact 
plate ;  the  levers  extend  to  the  back  of  the  gallows  arm,  and 
can  be  thrust  apart  or  brought  together  by  a  double  threaded 
screw,  turned  by  an  insulated  hand  wheel  for  the  purpose  of 
either  gripping  or  slackening  the  electrode.  All  three  contact 
plates  are  water-cooled  besides  the  back  and  front  door  frames ; 
water  jackets  are  also  provided  to  reduce  the  annular  openings 
between  the  electrodes  and  the  roof  brickwork.  The  furnace 
rolls  on  rockers,  and  is  tilted  by  a  heavy  tilting  bar  carrying  a  rack, 
which  is  held  in  mesh  with  a  driving  pinion  mounted  in  a  rigid 


278 


THE    ELECTRO-METALLURGY   OF    STEEL 


bearing.  The  vertical  electrodes  are  controlled  by  motor  or  by 
hand  through  a  rack  and  pinion  drive.  The  roof  consists  of  a 
steel  frame  lined  with  silica  brick,  and  rests  upon  the  furnace 
walls. 

Furnace  Lining. — There  is  no  special  feature  in  the  method 


of  building  either  a  basic  or  acid  lining.  The  walls  of  the  larger 
furnace  of  6  ton  capacity  are  18  inches  thick,  which  is  rather 
greater  than  in  other  types  of  furnaces.  The  basic  hearth  is 
rammed  in  with  a  hot  mixture  of  dead  burnt  magnesite  or 
dolomite,  15  per  cent,  of  basic  slag,  and  sufficient  roofing  pitch 
to  make  the  mixture  bind.  When  the  lining  is  complete,  the 


MODEKN  TYPES  OF  ELECTEIC  STEEL  FUENACES     279 

furnace  is  gradually  heated  by  means  of  an  arc  to  such  a  high 
temperature  that  the  hearth  becomes  fritted  in  place.  The 
hearth,  which  is  never  less  than  24  inches  thick,  is  frequently 
built  up  of  basic  material  fritted  in  layers  to  form  a  solid  homo- 
geneous mass.  There  is  nothing  distinctive  in  the  acid  lining, 
which  is  composed  of  the  usual  silica  brick  and  ganister 
materials ;  the  hearth,  however,  can  also  be  fritted  on  in  layers 
by  using  the  main  electrodes  in  conjunction  with  the  auxiliary. 

Electrodes. — Either  amorphous  or  graphite  electrodes  are 
used,  the  holder  being  so  designed  that  interchange  from  one  to 
the  other  type  is  easily  accomplished  by  substituting  contact 
plates  of  different  curvature. 

Furnace  Operation. — In  an  acid  lined  furnace  there  is  no 
conductive  bottom,  and  the  auxiliary  electrode  is  always  used 
as  a  common  return  for  the  main  electrodes ;  for  this  purpose 
it  is  kept  in  touch  with,  or  just  rests  upon  the  charge  until  melt- 
ing is  completed,  after  which  it  is  adjusted  to  remain  in  contact 
with  the  slag.  No  refining  other  than  that  of  carbon  control  is 
attempted  with  an  acid  bottom,  and  the  process  of  steel-making 
is  followed  in  the  ordinary  manner. 

When  starting  to  melt  a  cold  charge,  the  auxiliary  elec- 
trode mounting  is  released  by  a  clutch  attachment,  permit- 
ting it  to  rest  upon  the  scrap.  Direct  arcs  are  then  struck  by 
the  two  main  electrodes,  and  melting  begins  in  each  arc  zone. 
There  is  no  definite  melting  zone  under  the  auxiliary  electrode, 
which  remains  supported  by  the  unmelted  charge.  When 
melting  has  proceeded  to  such  an  extent  that  the  unmelted 
portion  begins  to  sink  into  the  bath  and  can  no  longer  carry 
the  weight  of  the  auxiliary  electrode,  the  latter  is  again  connected 
to  the  lifting  gear,  and  then  kept  just  in  contact  with  the  semi- 
fused  scrap  or  bath.  This  procedure  may  be  followed  both  in 
acid  or  basic  lined  furnaces,  only  in  the  latter  case,  the  hearth 
usually  becomes  sufficiently  conductive  to  carry  the  full  return 
current  before  the  charge  is  melted,  when  the  auxiliary  electrode  is 
raised  clear  of  the  charge.  The  furnace  then  operates  electrically 
in  the  same  manner  as  the  original  two-phase  Stobie  furnace, 
in  which  the  two-phase  circuits  are  independent.  The  Booth- 
Hall  furnaces  have  been  used  solely  for  melting  scrap  charges 
up  to  the  present  time,  the  high  voltage  arcs,  thick  linings  and 


280  THE   ELECTKO-METALLUEGY   OF    STEEL 

the  heavy  transformer  rating  being  especially  designed  for  this 
purpose.  The  radiation  loss  under  the  two  latter  conditions  is 
only  a  small  fraction  of  the  very  considerable  power  input,  as 
will  be  seen  from  the  ratings  of  the  li  and  3  ton  furnaces,  and 
the  thermal  efficiency  is  probably  about  85  per  cent,  at  full  load. 
The  power  consumption  per  ton  of  steel  should  be  rather  lower 
than  for  furnaces  less  heavily  rated.  A  small  1£  ton  furnace, 
working  about  eleven  hours  a  day,  has  averaged  about  670  units 
per  ton  of  steel  over  several  months,  which  is  a  very  good  per- 
formance for  a  basic  furnace  operating  intermittently. 

Ludlum  Furnace. — The  chief  features  of  this  furnace  are 
purely  constructional,  and  concern  more  especially  the  dis- 
position of  the  electrodes  and  the  shape  of  the  melting  chamber. 

Electrical  Design. — The  furnace  operates  off  a  simple  three- 
phase  low  tension  supply,  the  line  conductors  being  connected 
to  three  electrodes  which  enter  the  furnace  through  the  roof. 
The  open  circuit  line  voltages,  which  are  interchangeable,  are 
110,  95,  and  80. 

Furnace  Design. — The  three  electrodes  are  placed  in  a  line 
and  the  shape  of  the  body  so  designed  that  the  heat  radiated 
from  the  arc  zones  is,  as  far  as  possible,  of  uniform  intensity  at 
all  points  of  the  lining.  The  body  has  therefore  an  ellipsoidal 
shape,  and  is  provided  with  one  door  at  each  end  of  the  major 
axis.  The  roof,  which  is  of  varying  width  between  the  side 
walls,  has  a  constantly  changing  curvature,  but  is  said  to  be 
very  simple  to  build  and  provides  a  uniform  distribution  of  re- 
flected heat.  The  furnace  tilts  on  rockers  set  parallel  to  the 
major  axis,  the  pouring  spout  being  incorporated  in  one  of  the 
end  charging  doors.  The  electrode  columns  are  mounted  on 
one  side  of  the  furnace  shell  parallel  to  the  major  axis. 

The  furnace  is  suitable  for  working  either  the  acid  or  basic 
process,  as  the  hearth,  which  is  quite  independent  of  the  electri- 
cal circuits,  can  be  readily  lined  with  acid  material.  It  is  also 
interesting  to  note  that  graphite  electrodes  have  been  definitely 
found  to  be  more  economical  than  amorphous  by  users  of  this 
type  of  furnace. 


CHAPTEE  XIII. 

REFRACTORY  MATERIALS  AND  THEIR  APPLICATION  TO  ELECTRIC 
FURNACE  CONSTRUCTION. 

THE  economical  production  of  electric  steel  is,  above  all  other 
considerations,  dependent  upon  regularity  and  speed  of  opera- 
tion. Frequent  stoppages  for  furnace  repairs,  metallurgical 
difficulties  caused  by  tbe  chemical  action  of  failing  brickwork, 
and  excessive  loss  of  heat  through  prematurely  worn  linings, 
may  all  be  credited  to  the  failure  of  the  refractory  materials  used 
in  furnace  construction.  It  would  not,  however,  be  correct  in 
all  cases  to  assign  failure  of  the  lining  to  the  inferior  quality  of 
the  material  used,  since  heat  developed  by  the  electric  arc  is  so 
intense  that  by  ill-considered  application  it  may  easily  cause 
complete  breakdown  of  the  best  refractory  materials  procur- 
able. Of  recent  years  special  refractories,  such  as  fused  alumina, 
carborundum  and  zirconia,  have  been  suggested  for  use  in  the 
electric  furnace  in  the  form  of  bricks,  as  it  is  admitted  that 
both  silica  and  magnesite  do  not  meet  the  very  exacting  demands 
occasioned  by  the  peculiar  conditions  of  electric  furnace  opera- 
tion. 

Classification. — Kefractory  materials  are  broadly  classified 
according  to  their  powers  of  resisting  the  corrosive  action  of 
siliceous  and  basic  slags.  Materials  in  which  silica  predominates 
are  acid,  while  those  composed  essentially  of  such  powerful 
bases  as  lime  and  magnesia  are  basic  ;  others,  composed  of 
oxides  or  compounds  that  are  attacked  equally  by  both  acid  and 
basic  fluxes  alike,  or  remain  unattacked  by  either,  are  called 
"neutral  ".  Considerable  care  has  to  be  exercised  in  the  selec- 
tion of  refractories  and  their  treatment  both  before  and  during 
service,  and  in  this  chapter  such  information  as  is  given  bears 
more  especially  upon  their  use  in  electric  furnaces. 

Conditions  of   Service. — Before  considering   the  properties, 

(281) 


282  THE    ELECTRO-METALLURGY   OF    STEEL 

composition,  and  behaviour  of  the  various  refractories  commonly 
used,  it  is  advisable  to  consider  the  nature  of  the  chemical, 
physical,  and  mechanical  conditions  to  which  they  are  exposed. 

Chemical  corrosion  by  the  action  of  slag  is  not  so  con- 
fined to  the  banks  as  in  the  open-hearth  furnace,  but  is  pro- 
moted also  in  the  region  of  the  walls  and  roof  by  the  action  of 
fluxes,  especially  powdered  lime,  and  by  slag  which  may  be 
violently  thrown  upwards  from  the  bath  on  the  addition  of  damp 
scrap,  or  for  other  reasons.  The  high  temperature  of  the  slag 
also  exerts  a  powerful  influence  upon  its  corrosive  action  on  the 
lining,  which  may  be  further  intensified  by  only  slight  deviation 
from  its  correct  composition  as  regards  degree  of  basicity  or 
the  amount  of  iron  oxide  present. 

The  requirements  imposed  by  the  physical  conditions  are 
essentially  those  which  demand  the  highest  power  of  resist- 
ance to  change  of  state  under  the  influence  of  an  intense  and 
sometimes  abruptly  changing  temperature.  In  the  case  of 
indirect  arc  furnaces,  the  roof  and  walls  are  more  exposed  to  the 
direct  radiation  of  heat  from  the  arc  than  is  the  case  with  direct 
arc  furnaces,  but  even  in  the  latter  certain  portions  of  the 
lining,  under  the  influence  of  drawn-out  or  laterally  deflected 
arcs,  will  be  exposed  to  intense  local  heating. 

The  internal  mechanical  stresses  set  up  by  temperature 
variation  are,  in  the  case  of  electric  furnace  linings,  exceedingly 
severe.  Almost  all  refractory  materials  expand  on  heating,  so 
that  frequent  changes  of  temperature  accompanied  by  alternate 
expansion  and  contraction  are  liable  to  cause  rupture.  It  is  the 
usual  practice  in  electric  furnace  roof  construction  to  build  the 
silica  brickwork  in  a  rigid  steel  frame,  and,  although  special 
precautions  are  taken  to  allow  for  expansion,  the  risk  of  distor- 
tion, which  may  lead  to  crushing  of  the  brickwork,  will  be 
greater  than  in  the  case  of  large  gas  furnace  roofs  which  are 
freer  to  rise  and  fall  under  less  frequent  variations  of  tempera- 
ture. Kapid  heating  causes  unequal  expansion,  and  internal 
mechanical  stresses  are  set  up  which  cause  rupture  unless  the 
tensile  strength  of  the  agglomerated  material  is  sufficiently  high. 

The  charging  and  working  doors  of  electric  furnaces  are 
generally  made  as  small  as  is  consistent  with  the  necessary  ease 
of  charging,  fettling,  and  withdrawal  of  broken  electrodes,  so 


REFRACTORY  MATERIALS  AND  FURNACE  CONSTRUCTION   283 

that  the  door  jambs  and  arches  are  always  liable  to  rough  usage 
and  injury  from  the  various  tools  used  for  manipulation.  Un- 
fortunately, it  is  just  the  brickwork  round  the  door  openings 
that  suffers  most  from  temperature  changes,  so  that  it  is  diffi- 
cult to  find  a  satisfactory  material  that  will  resist  such  a  com- 
bination of  severe  conditions. 

After  indicating  the  severity  of  the  working  conditions  that 
have  to  be  faced,  it  is  now  possible  to  consider  the  properties 
and  suitability  of  various  refractory  materials,  classified  accord- 
ing to  their  character  as  previously  defined. 

Basic  Materials. — Dolomite  and  magnesite  are  the  only  basic 
materials  used  either  for  the  manufacture  of  bricks  or  for  the 
construction  of  furnace  hearths. 

Dolomite. — The  mineral  dolomite  may  be  regarded  as  a 
crystalline  limestone  in  which  part  of  the  calcium  carbonate  is 
replaced  by  magnesium  carbonate.  It  becomes  exceedingly  hard 
and  tough  after  calcination  at  a  high  temperature,  and  in  this 
state  has  only  a  moderate  tendency  to  slake  when  exposed  to 
a  moist  atmosphere.  Before  it  is  suitable  as  a  refractory 
material,  it  requires  to  be  burnt  or  calcined  until  all  the  carbon 
dioxide  has  been  expelled.  During  this  calcination  the  raw 
material  shrinks  and,  if  properly  burnt,  will  show  signs  of 
incipient  fusion. 

The  composition  of  dolomite  before  and  after  burning  may 
be  given  by  the  following  typical  analyses : — 

Constituents.  Per  Cent,  before  Burning.  Per  Cent,  after  Burning. 

Carbon  dioxide   .         .         .    44-46  1-0 

FeO  and  A12O,   .        .        .       1-2  3-1 

CaO 32-0  57-3 

MgO  .....     20-0  37-4 

SiOj  •        .         .        .       1-0  2-0-3-5 

Burnt  dolomite  is  best  purchased  and  stored  in  the  lump 
condition,  and  only  crushed  to  the  required  size  shortly  before 
use.  Sometimes  it  is  bought  already  crushed  and  barrelled,  but 
this  is  not  to  be  recommended,  as  it  is  more  liable  to  perish  or 
slake  during  transit  and  on  prolonged  standing.  It  should  be 
remembered  that  the  fine,  white,  slaked  powder  present  in 
crushed  dolomite,  that  has  been  stored  for  any  length  of  time, 
contains  water  of  hydration,  and  if  such  material  were  used  for 


284  THE    ELECTRO-METALLURGY   OF    STEEL 

forming  a  furnace  bottom  the  water  would  subsequently  be 
driven  off  and  destroy  the  solidity  of  the  rammed  material. 
Power  of  heat  resistance  will  depend  on  the  amount  of  silica 
and  other  oxides  present,  which  tend  to  increase  the  fusi- 
bility by  the  formation  of  multi-base  silicates. 

Dolomite  is  usually  reserved  for  the  construction  of  the 
working  hearth,  but  can  be  more  extensively  used  in  the  lining 
of  small  furnaces  ;  in  the  latter  case  entire  walls  composed  of 
rammed  dolomite  have  given  perfect  satisfaction. 

Magnesite. — Magnesiteis  the  only  material  that  can  be  satis- 
factorily used  for  the  manufacture  of  basic  bricks.  The  mineral, 
which  in  its  raw  state  consists  of  magnesium  carbonate  contain- 
ing small  and  varying  amounts  of  Si02,  A1203,  and  Fe203,  is  ex- 
tensively mined  in  Styria,  Greece,  Asia  Minor,  and  India.  Like 
dolomite  it  requires  to  be  burnt  at  a  very  high  temperature,  not 
,only  to  expel  the  carbon  dioxide  from  the  carbonates,  but  also 
to  convert  the  resulting  amorphous  magnesia  (MgO)  into  a 
crystalline  state.  The  proportion  of  ferric  oxide  present  in 
the  raw  material  is  considered  of  far  greater  importance  than 
silica,  as  it  gives  a  much  greater  binding  strength  to  the  particles 
of  a  burnt  brick  without  impairing  its  power  of  heat  resistance. 

There  is  no  doubt  that  bricks  made  from  Austrian  magnesite 
have  given  better  results  than  others,  notwithstanding  the  fact 
that  the  burnt  material  contains  a  larger  percentage  of  impurities 
than  varieties  of  inferior  physical  quality.  The  following  are 
typical  analyses  of  burnt  Grecian  and  Austrian  magnesite  l  :— 

Constituent.  Grecian.  Austrian. 

MgO 88-0  per  cent.  83-0  per  cent. 

CaO 5-0       ,,  4-0       ,, 

A1203 1-0       „  3-0       „ 

Fe203  ....         -5       „  8-0       „ 

Si02 5-5       „  2-0       „ 

Amorphous  magnesium  oxide  becomes  crystalline  at  the 
temperature  normally  attained  in  electric  furnaces,  and  in  so 
doing  undergoes  considerable  shrinkage.  This  is  the  chief 
cause  of  "  spalling,"  which  leads  to  the  rapid  destruction  of 
magnesite  walls,  and  for  this  reason  raw  magnesite  should  be 
burnt  at  the  highest  possible  temperature  until  it  becomes  at 

]  Transactions  of  the  Faraday  Society,  June,  1917. 


REFRACTORY   MATERIALS   AND  FURNACE   CONSTRUCTION      285 

least  partially  crystalline.  The  Austrian  "Spaeter"  brick  has 
a  dark,  finely  crystalline  structure,  and  is  somewhat  heavier 
than  English  varieties  ;  the  greater  density,  although  no  doubt 
partly  due  to  the  higher  percentage  of  iron  oxide,  results 
from  the  more  perfect  conversion  of  the  amorphous  magnesia  to 
the  crystalline  state. 

Magnesite,  which  is  burnt  either  in  cupolas  or  in  gas  furnaces, 
is  generally  crushed  rather  smaller  than  dolomite  before  calcina- 
tion. Cupola-burnt  magnesite  requires  to  be  carefully  hand 
picked,  and  all  semi-burnt  pieces  are  returned  for  re-calcination. 
Burnt  magnesite  may  be  used  in  place  of  dolomite  for  lining 
furnace  hearths,  but  it  is  doubtful  whether  any  advantage  can 
be  gained  in  cases  where  it  is  not  possible  to  build  up  the  hearth 
by  fritting  on  thin  successive  layers  of  the  burnt  material,  mixed 
with  a  suitable  chemical  binder  such  as  basic  slag.  For  electric 
steel  furnace  use  there  is  no  question  that  dolomite  alone  can 
be  employed  with  perfect  success,  provided  the  correct  methods 
of  mixing  and  ramming  are  followed. 

So  as  to  judge  the  suitability  of  magnesite  bricks  for 
electric  furnace  use,  it  is  well  to  consider  their  physical  pro- 
perties and  the  temperature  and  mechanical  conditions  they  are 
called  upon  to  resist.  Magnesite  bricks  have  a  high  coefficient 
of  thermal  conductivity,  and  in  order  to  prevent  excessive  heat 
loss,  they  should  be  backed  wherever  possible  by  a  material 
having  a  much  higher  power  of  heat  insulation.  In  most  basic 
lined  electric  furnaces  the  basic  brick  lining  is,  however,  only 
continued  a  few  courses  above  the  slag  line,  so  that  this  pre- 
caution need  hardly  be  followed  where  the  walls  are  only  built 
9  inches  thick.  Magnesite  bricks  behave  admirably  so  far  as 
they  resist  fusion  at  very  high  temperatures  and  corrosion 
by  basic  slags,  and  the  only  serious  objection  to  their  use  is 
their  friability  when  hot  and  their  tendency  to  burst  or  "  spall  " 
after  exposure  to  very  high  temperatures.  This  serious  draw- 
back has  been  studied  by  numerous  investigators  and,  according 
to  the  general  opinion  formed,  is  largely  due  to  imperfect  conver- 
sion of  the  amorphous  magnesite  to  its  crystalline  oxide  during 
the  burning  operation.  Magnesite  bricks  when  used  in  electric 
furnaces  are  exposed  to  temperatures  which  rapidly  promote 
this  physical  change,  accompanied  by  shrinkage,  so  that  fracture 


286  THE    ELECTRO-METALLURGY   OF    STEEL 

generally  occurs  along  a  plane  dividing  the  unaltered  from  the 
altered  material.  Those  indirect  arc  furnaces,  in  which  it  is 
imperative  to  use  magnesite  brick  instead  of  silica  for  the  roof 
construction,  are  placed  at  a  serious  disadvantage  in  this  re- 
spect. 

Neutral  Materials. — The  only  neutral  refractories  commonly 
used  are  bauxite  and  chromite,  but  considerable  work  has 
recently  been  done  on  the  study  of  fused  alumina  or  "  alundum  " 
and  zirconia  for  use  in  electric  furnace  linings. 

Bauxite. — Bauxite  is  found  as  a  mineral  containing  45  per 
cent,  to  75  per  cent.  A1203  in  the  form  of  a  hydrated  oxide  to- 
gether with  varying  amounts  of  iron  oxide  and  silica.  For 
use  as  a  refractory  material  bauxite  should  contain  as  much 
alumina  as  possible  while  the  impurities  should  be  correspond- 
ingly low.  The  analysis  below  is  that  of  a  high  grade  sample 
of  French  bauxite  : — 

A1203    .....  781    peif  cent. 

Fe203 1-02 

SiO2 5-78 

Water 1510 

The  raw  material  is  first  calcined  to  dehydrate  the  oxides 
present,  then  mixed  with  high-grade  clay  to  form  a  plastic 
mixture,  and  finally  pressed  into  the  form  of 'bricks.  The  re- 
fractory quality  of  bauxite  is  considerably  influenced  by  the 
nature  and  the  amount  of  the  impurities  present,  which  may 
form  double  and  triple  silicates  of  lower  melting-point  than  pure 
aluminium  silicate.  The  best  quality  bricks  have  a  coarse- 
grained fracture,  similar  to  a  fire-brick  but  nearly  white.  Iron 
oxide  colours  the  brick,  from  a  pale  yellow  to  a  light  red,  ac- 
cording to  the  amount  present.  The  inferior  grades  behave 
best  in  an  oxidising  atmosphere,  and  when  exposed  to  reducing 
gases  become  less  refractory  owing  to  the  reduction  of  the  ferric 
oxide  to  ferrous  oxide,  which  is  not  only  more  fusible  but  readily 
combines  to  form  double  silicates. 

Bauxite  bricks  of  the  highest  quality  provide  an  excellent 
material  for  furnace  linings,  but,  unfortunately,  their  extended 
use  is  limited  by  their  high  cost.  They  have  a  low  co- 
efficient of  expansion,  and  are  quite  unaffected  by  the  most 


REFRACTORY  MATERIALS  AND  FURNACE  CONSTRUCTION   287 

abrupt  changes  of  temperature;  the  neutral  behaviour  of 
bauxite  enables  it  to  be  placed  in  contact  with  either  silica  or 
magnesite  bricks,  so  that  it  can  be  used  as  a  neutral  parting  in 
special  cases.  The  finest  bauxite  will  resist  higher  temperatures 
than  silica,  and  although  costly,  may  sometimes  prove  more 
economical  for  roofs  of  high  temperature  furnaces  operating 
intermittently.  Bauxite  bricks  are,  however,  liable  to  be  fluxed 
by  fumes  of  metallic  oxides  or  lime,  so  that  their  value  as  a 
refractory  is  greatly  reduced  in  basic  lined  furnaces.  Patching 
cement,  of  the  same  material  as  is  used  for  pressing  into  bricks, 
is  a  most  useful  and  better  substitute  for  magnesite  or  ganister, 
since  it  is  more  plastic  and  more  readily  adheres  to  damaged 
brickwork.  Although  bauxite  resists  chemical  fusion  with  either 
basic  or  acid  refractories,  it  must  not  be  assumed  that  it  is 
equally  inert  to  the  action  of  slags  which,  however  acid  or 
basic,  contain  silicates  and  therefore  readily  combine  with  the 
alumina  (A1203)  to  form  more  complex  silicates. 

Alundum. — Alumina  as  a  refractory  material  has  so  far  been 
considered  in  its  amorphous  state.  Considerable  development 
has  been  made  in  the  preparation  and  use  of  fused  crystalline 
alumina,  which  in  moulded  form  is  known  as  "  Alundum  ".  This 
refractory  has  been  used  for  electric  furnace  roofs  in  America, 
but  it  is  said  that  difficulties  are  encountered  owing  to  the 
action  of  lime  vapours  arising  from  the  intensely  heated  basic 
slag.  This  action  will  be  accentuated  by  the  silica  present, 
which  enters  the  brick  as  a  constituent  of  the  binder  employed. 

Chromite. — This  mineral,  consisting  of  mixed  chromium  and 
iron  oxides,  is  used  to  a  limited  extent  as  a  refractory  material, 
and  resists  fusion  at  exceedingly  high  temperatures. 

The  crude  ore,  when  crushed  to  pass  about  J  inch  screen,  is 
sometimes  employed  as  a  parting  between  magnesite  and  silica 
brickwork ;  this  practice  was  at  one  time  largely  followed  in 
lining  basic  open  hearth  furnaces,  but  has  now  been  practically 
given  up.  When  used  in  electric  furnaces  there  is  always  the 
danger  of  reduction  should  any  become  displaced  and  enter  the 
highly  reducing  basic  slag.  Chromite  bricks  are  very  heavy  and 
mechanically  weak,  and  are  seldom  used. 

Acid  Materials. — Silica. — Silica  is  the  predominating  con- 
stituent of  all  acid  refractories,  and  ranges  from  98*5  percent,  in 


288  THE   ELECTKO-METALLtJKGY   OF    STEEL 

the  Sheffield  "black  ganister"  to  80  per  cent,  in  the  common 
grade  of  "  cupola  "  ganister.  Silica  rock,  containing  over  97  per 
cent,  of  SiO2,  is  generally  used  for  the  manufacture  of  bricks, 
whereas  the  various  grades  of  ganister  containing  rather  less 
silica  are  usually  employed  for  ramming  and  patching.  The 
higher  the  Si02  the  less  will  be  the  tendency  to  bind,  either  be- 
fore or  after  burning,  so  that  in  the  manufacture  of  bricks  from 
the  purest  quartzite  about  1|  per  cent,  of  lime  is  added,  which 
frits  together  the  quartz  particles  during  the  burning  operation 
to  form  a  concrete  mass.  It  is  well  known  that  silica  can  exist 
in  several  different  forms  according  to  the  temperature  to  which 
it  has  been  exposed. 

Quartzose  silica,  as  used  for  the  manufacture  of  silica  bricks, 
has  a  specific  gravity  of  2 '65  which  slowly  falls  to  2 '3  under 
the  action  of  heat,  which  at  the  same  time  causes  a  slow  trans- 
formation to  an  allotropic  variety.  The  complete  change  is 
equivalent  to  an  expansion  of  16  per  cent.,  which  is,  however, 
partly  counteracted  by  the  natural  shrinkage  that  accompanies 
incipient  fusion  at  the  highest  furnace  temperatures.  Once 
this  transformation  is  complete,  the  degree  of  expansion  and 
contraction  resulting  from  temperature  changes  will  become 
far  less  marked.  This  explanation  falls  in  with  the  observed 
fact  that  a  well-seasoned  roof  is  far  less  sensitive  to  changes 
of  temperature  than  when  new.  The  importance  of  burning 
silica  bricks  at  the  highest  kiln  temperature  possible  is  evident, 
as  by,  this  treatment  the  resistance  to  "spelling  "will  be  in- 
creased. The  softening  or  fusing  point  of  silica  bricks  has  been 
variously  given  as  lying  between  1650°  C.  and  1800°  C.,  so  that 
great  care  must  be  taken  in  the  operation  of  electric  furnaces 
in  which  higher  temperatures  are  so  easily  and  accidentally 
reached.  Fusion  at  normal  casting  temperatures  is  not  such  a 
serious  factor  as  the  fluxing  action  of  basic  dust  and  heavy 
oxide  fumes,  which  together  form  fusible  double  silicates  and 
consume  the  brickwork  by  chemical  action. 

The  greatest  trouble  to  be  met  with  in  the  use  of  silica 
bricks  is  their  tendency  to  "  spall  ".  "  Spalling  "  is  partly  due 
to  stresses  set  up  by  unequal  expansion  due  to  rapid  heating, 
alternate  expansion  and  contraction,  and  compression  caused  by 
the  considerable  expansion  of  untransformed  quartzose  silica. 


BEFRACTOEY  MATERIALS  AND  FURNACE  CONSTRUCTION   289 

Unless  sufficient  provision  is  made  to  allow  for  expansion  during 
the  initial  heating,  silica  bricks  will  be  unable  to  withstand  the 
crushing  stresses  and  burst.  It  must  be  admitted  that  the  roof 
has  to  withstand  harsher  treatment  in  electric  than  in  gas 
furnaces,  where  mechanical  provision  is  more  easily  made  to 
allow  for  this  expansion.  For  this  reason  only  the  best  silica 
bricks  should  be  used,  and  great  care  taken  not  to  hasten  the 
initial  drying  out  and  heating  to  the  full  furnace  temperature. 
For  the  first  two  or  three  heats  the  lining  should  be  carefully 
nursed,  and  never  exposed  to  an  unnecessarily  high  temperature 
until  properly  glazed  and  seasoned. 

The  grain  size  of  silica  bricks  and  the  degree  of  burning  are 
just  as  important  as  their  chemical  composition,  since  these 
factors  alone  influence  their  resistance  to  "  spalling  ".  As  a 
general  rule  a  high  quality  brick  will  have  a  very  pale  yellow 
fracture,  showing  a  structure  composed  of  numerous  sharp 
irregular-shaped  particles  of  various  sizes,  closely  bound  together 
in  a  fine  grained  matrix.  The  presence  of  small  cavities  is  not 
detrimental.  Bricks  coloured  by  red  oxide  of  iron  may  behave 
well  in  oxidising  atmospheres,  but  are  not  suitable  for  electric 
furnaces  in  which  highly  reducing  gases  are  generated.  They 
should  be  mechanically  strong  and  ring  when  struck ;  large 
blocks  are  seldom  properly  burnt  in  the  centre,  and  this  point 
should  be  considered  when  designing  special  roof  bricks  or  other 
special  shapes.  Silica  bricks  readily  absorb  water  owing  to 
their  porosity,  and  should  always  be  stored  under  cover.  Any 
moisture  absorbed  by  a  brick  is  vaporised  in  heating,  but  may 
only  escape  with  difficulty,  so  that  the  tendency  to  "  spall "  or 
burst  is  greatly  increased  unless  special  precautions  are  taken 
to  dry  out  very  slowly.  Some  brick-makers  consider  that 
moisture  is  not  completely  expelled  below  a  red  heat,  and  if  this 
be  the  case,  it  is  not  safe  to  assume  that  a  roof  is  perfectly 
dehydrated  when  steam  ceases  to  rise. 

Ganister. — The  term  "  Ganister  "  applies  more  properly  to 
silica  rock  of  lower  silica  content  than  is  used  for  the  manu- 
facture of  bricks,  the  so-called  "  black  ganister  "  being  a  true 
quartzite.  Ganister  rock,  when  crushed  and  moistened,  is 
slightly  plastic  owing  to  the  small  percentages  of  alumina  and 
lime  present  as  silicates.  The  plasticity  of  the  less  refractory 

19 


290 


THE    ELECTRO-METALLUKGY   OF    STEEL 


grades  of  ganister  increases  with  the  percentage  of  these  natural 
silicates. 

Crucible  ganister  is  a  high  grade  ganister  rock  with  just 
sufficient  plasticity  to  enable  it  to  hold  together  when  rammed, 
and  is  used  for  lining  crucible  steel  pot  holes.  This  grade  is 
very  light  in  colour  and  is  a  very  useful  material  for  patching 
silica  brick  linings  and  door  jambs.  It  is  also  used  for  building 
up  the  hearths  of  acid  lined  electric  furnaces,  and  is,  at  the  same 
time,  most  useful  as  a  fettling  material. 

Cupola  ganister  is  the  least  refractory  grade,  and  is  only 
used  for  making  up  furnace  launders  or  spouts,  and  in  other 
places  that  are  not  exposed  to  very  high  temperatures. 

Silica  Cement. — This  material  is  used  for  bedding  silica 
bricks,  and  generally  consists  of  a  finely  ground  mixture  of  old 
silica  bricks  and  fresh  quartzite  rock  to  which  a  small  quantity 
of  a  lower  grade  ganister  is  added  to  make  it  sufficiently  plastic. 
Fire-clay  is  sometimes  used  as  a  substitute  for  the  ganister,  but 
this  practice  destroys  the  refractory  properties  of  the  cement 
and  is  not  to  be  recommended. 

Silica  Sand. — Highly  refractory  sand,  approaching  pure 
silica  as  nearly  as  possible,  is  used  for  fettling  the  banks  of  large 
electric  furnaces.  In  small  furnaces  the  banks  are  usually  too 
steep  and  ganister  is  then  used.  Originally,  silver  sand  for  this 
purpose  was  imported  from  Belgium  and  Holland  but,  when 
these  sources  of  supply  failed,  English  sands  were  used,  which 
have  given  equally  good  results. 

Analyses  of  Acid  Materials.— Typical  analyses  of  silica 
bricks  and  other  acid  materials  are  given  below  :— 


Si02. 

A1203- 

Fe203. 

CaO. 

Alkalies. 

MgO. 

L  oss  on 
Ignition. 

Dinas  silica  brick 

96-8 

•92 

•5 

1-2 

•2 

Yorkshire  silica  brick 

96-2 

•92 

•63 

1-5 

•19 

•22 



Crucible  ganister 

95-3 

2-10 

•80 

1-10 

•28 

•23 



Scotch  silica  sand 

99-4 

•6 

— 

— 

— 

— 



Fire-clay. — Although  fire-clay  is  not  commonly  regarded  as 
an  acid  refractory  its  analysis  brings  it  within  this  category. 
Few  fire-clay  bricks  are  capable  of  withstanding  the  tempera- 


REFRACTORY  MATERIALS  AND  FURNACE  CONSTRUCTION   291 


tures  of  steel  furnaces,  so  that  they  are  only  used  in  a  limited 
extent  as  a  backing  to  either  magnesite  or  silica  bricks  and  for 
lining  doors.  Fire-clay  is,  however,  extensively  used  for  lining 
ladles  and  in  the  ingot  pit,  so  that  it  deserves  mention  when 
dealing  generally  with  the  various  refractories  used  in  the 
manufacture  of  steel. 

Fire-clays  consist  essentially  of  alumina  and  silica  with  small 
and  varying  amounts  of  iron  oxide,  lime,  magnesia,  and  alkalies, 
which  are  usually  present  as  impurities.  Neglecting  the  effect  of 
impurities,  there  is,  according  to  Prof.  H.  Le  Chatelier,  a  definite 
compound  of  alumina  and  silica,  consisting  of  approximately 
15  molecules  of  Si02  and  1  molecule  A12O3,  which  has  the  lowest 
melting-point,  and  any  increase  of  either  the  Si02  or  the  alumina 
content  above  these  proportions  increases  the  refractoriness. 
At  one  end  of  the  scale  there  is  pure  alumina,  and  at  the  other 
pure  silica,  the  latter  having  the  lower  melting-point.  This 
serves  as  a  rough  guide  for  judging  the  working  properties  of 
a  fire-brick  from  its  analysis,  other  conditions  being  equal. 

The  behaviour  of  fire-bricks  will  depend,  like  silica,  upon 
their  structure,  chemical  analysis  and  conditions  of  service,  and 
they  must  be  selected  for  any  particular  use  from  the  results  of 
practical  experience.  For  lining  furnace  doors,  the  finest  brick 
should  be  used  to  avoid  constant  renewal  and  badly  fluxed  door 
sills. 

Fire-clay,  whether  used  for  bedding  bricks  or  for  ingot  pit 
work,  is  mixed  with  water  and  allowed  to  "  temper  "  several 
days  before  use,  otherwise  it  does  not  hold  the  water  so  well  and 
is  not  sufficiently  plastic. 

A  few  typical  analyses  of  fire-clays  and  fire-bricks  are  given  in 
the  following  table  : — 1 


Si02. 

A1A- 

Fe203. 

CaO. 

MgO. 

Alkalies. 

Loss  on 
Ignition. 

H^O. 

Stourbridge  fire-clay 

65-0 

22-0 

2-0 

•5 



•5 

10-0 

_ 

Stourbridge  fire-brick 

69-0 

27-3 

1-86 

•27 

•32 

•91 

— 

— 

Glenboig  fire-clay 

62-0 

28-0 

2-0 

•5 

•5 

•5 

6-5 

— 

Glenboig  fire-brick    . 

63-0 

32-0 

2-85 

•79 

•36 

•94 

— 

— 

Transactions  of  the  Faraday  Society,  June,  1917. 


CHAPTEE  XIV. 

FUENACE  LINING  AND  LINING  REPAIRS. 

THE  materials  used  in  the  hearth  construction  of  electric  furnaces 
will  be  either  basic  or  acid,  according  to  the  nature  of  the  pro- 
cess employed.  Basic  and  acid  linings  are  best  considered 
independently,  the  roof  and  upper  portion  of  the  walls  being 
substantially  the  same  in  both  cases.  The  methods  of  hearth 
construction  also  depend  upon  the  presence  or  absence  of  bottom 
electrodes,  so  that  further  subdivision  is  necessary.  The  follow- 
ing methods  are  those  generally  adopted,  but  may  be  slightly 
modified  to  suit  certain  types  of  furnaces,  of  which  special 
mention  has  already  been  made. 

Basic  Linings  without  Bottom  Electrodes. — The  brick  lining 
immediately  next  to  the  shell  plates  is  generally  built  of  best 
quality  fire-bricks,  which  may  be  carried  up  from  the  bottom  to 
a  point  beyond  which  there  would  be  a  possible  risk  of  contact 
with  slag.  From  that  point  upwards  magnesite  bricks  must  be 
used  for  the  purpose  of  resisting  the  corrosive  action  of  basic 
slag  in  the  event  of  it  penetrating  the  dolomite  hearth  lining. 
The  magnesite  bricks  are  always  built  up  to  a  level  at  least  two 
courses  above  the  charging  door  sill,  so  that  no  serious  injury 
can  be  done  to  the  uncovered  walls  by  the  corrosive  action  of 
oxidised  scrap,  which  may  rest  against  them  during  the  melting 
operation.  Silica  bricks,  of  the  best  quality  procurable,  are 
used  for  the  upper  part  of  the  wall  lining  and  roof.  In  the  case 
of  furnace  walls  thicker  than  9  inches  the  magnesite  and  silica 
wall  bricks  are  usually  backed  with  fire-brick,  and  in  some  cases 
the  magnesite  courses  will  be  only  4|  inches  thick  in  place  of 
the  full  9  inches,  which  is  more  usually  preferred.  It  was  at 
one  time  customary  to  use  magnesite  bricks  in  place  of  fire-bricks 
for  lining  the  bottom  and  lower  walls,  but  this  practice,  which 
was  far  more  expensive  and  resulted  in  greater  heat  losses,  owing 

(292) 


FtJBNACE   LINING  AND  LINING  REPAIRS  293 

to  the  better   heat   conductivity  of  magnesite,  has   now  been 


\L 


J/ 


CO 


given  up.     The  thickness  of  brickwork  on  the  bottom  and  next 
to  the  side  shell  plates  varies  according  to  the  design  of  furnace, 


294  THE   ELECTRO-METALLURGY.    OF   STEEL 

but  the  general  disposition  of  the  magnesite,  fire-clay  and  silica 
bricks  will  in  all  cases  be  alike. 

Three  successive  stages  in  lining  a  furnace  are  shown  in 
Fig.  124,  a  plan  and  transverse  section  being  given  to  illustrate 
the  brickwork  at  each  stage.  The  bottom  plate  is  first  covered 
with  a  course  of  best  quality  fire-brick  carefully  bedded  and 
bonded  to  obtain  the  best  solidity  possible,  as  shown  in  the 
sketch  B.  The  fire-brick  is  continued  upwards,  as  shown  in 

C,  to  form  a  flat  seating,  upon  which  the  magnesite  walls  are 
then  built ;  two  short  -£  inch  iron  rods  penetrate  the  brickwork 
so  as  to  ensure  electrical  contact  between  the  shell  plate  and 
a  hot  and  conductive  portion  of  the  dolomite  bottom,  when  the 
furnace  is  in  operation.     The  walls  of  magnesite  brick  are  then 
built  with  the  aid  of  a  carefully  centred  trammel,  as  indicated  in 

D,  up  to  a  level  two  courses  above  the  charging  door  sill ;  beyond 
this  point  there  is  no  risk  of  any  corrosive  action  of  rusty  scrap 
or   slag   on    the    silica   brick.     Magnesite    bricks    are    usually 
bedded  with  a  magnesite  slurry,  which  consists  of  finely  ground 
magnesite  bricks  mixed  with  just  enough  clay  to  make  it  slightly 
plastic.     They   should  be  set   as  close   as  possible,  using  only 
sufficient  slurry  to  secure  tight  joints.     The  silica  brick  walls 
are  carried  upwards  from  the  top  magnesite  course  without  any 
neutral  parting,  the  bricks  being  bedded  in  silica  cement  and  all 
joints  made  as  close  as  possible.     Kammed  dolomite  has  also 
been  used  with  success  for  the  construction  of  walls  of  small 
furnaces,  bricks  being  used  for  the  door  openings  only.     It  is 
better  not  to  bond  the  door  jambs  to  the  wall  lining,  otherwise 
their  renewal  or  repair,  which  is  usually  necessary  once  during 
the  lifetime  of  a  lining,  is  more  difficult  to  carry  out  without  risk 
of  damage  to  the  walls   themselves.     The   top   two   or   three 
courses  of  silica  brick  are,  in  some  cases,  stepped  forward  to 
offer  greater  protection  to  the  lower  angle  of  the  roof   frame, 
which  supports   the  skewback   blocks.     A   method    frequently 
adopted  for  constructing  walls  of  14  inch  thickness  in  the  case 
of  furnaces  of  7  tons  capacity  and  upwards,  is  shown  in  Fig. 
125. 

Before  putting  in  the  dolomite  bottom,  the  brick  lining 
should  be  thoroughly  dried  out,  to  prevent  the  possibility  of 
moisture  coming  into  contact  with  the  dolomite  during  or  after 


FURNACE   LINING  AND   LINING  REPAIfcS 


295 


the  process  of  baking.  This  drying  out  can  be  done  either 
before  the  silica  walls  are  begun,  or  after,  as  may  be  most 
convenient.  The  furnace,  after  drying  out  and  cleaning,  is 
ready  to  receive  the  dolomite  hearth. 

The  hearth,  composed  of  rammed  dolomite,  is"  the  most  im- 
portant part  of  the  lining,  and  has  to  be  formed  with  great  care 
to  ensure  uniformity  and  perfect  solidity.  Imperfect  ramming 
or  the  use  of  bad  material  may  lead  to  serious  trouble,  and  in 
extreme  cases  to  a  break-out.  Considerable  attention  is  given, 
therefore,  to  the  preparation  of  the  dolomite  and  the  subsequent 
mixing  with  a  binder,  which  consists  of  tar  and  pitch  in  suitable 
proportions.  Such  a  mixture  is  commonly  called  "  Black 


FIG.  125. 

Basic  "  in  open-hearth  and  converter  plants,  where  it  is  usually 
prepared  in  special  mills  and  mixers.  Unfortunately,  the  size 
of  most  electric  furnace  plants  does  not  justify  the  installation 
of  special  plant  for  preparation  of  the  "black  basic,"  and  there- 
fore recourse  is  made  to  hand-mixing,  which,  to  be  conducted 
successfully,  entails  much  hard  work  and  patience.  If  a  crush- 
ing mill  is  available,  the  dolomite,  preferably  stored  in  lump 
form,  should  be  freshly  crushed  to  pass  a  |  inch  or  f  inch  mesh 
sieve,  while  sufficient  "  smalls  "  and  dust  should  be  made  when 
crushing  to  form  a  perfectly  solid  matrix  for  the  larger  pieces 
when  rammed.  The  correct  porportion  of  large  to  small  is 
soon  determined  by  experience,  and  then  there  should  be  no 
further  difficulty  in  obtaining  a  uniform  grading.  Partly  slaked 


296  THE   ELECTBO-METALLtJKGY  OF   STEEL 

dolomite  should  not  be  used,  as  the  moisture  in  the  dust,  which 
helps  to  form  the  matrix,  is  later  driven  off,  and  the  solidity  of 
the  material  is  then  destroyed. 

The  crushed  dolomite  is  heated  to  about  100°  C.,  and  then 
mixed  with  a  small  quantity  of  binder.  The  correct  proportions 
of  boiled  anhydrous  tar  and  hard  black  pitch,  of  which  the  binder 
is  composed,  are  best  found  by  trial  in  each  case,  as  the  viscosity 
and  melting-points  of  boiled  tar  and  pitch  are  very  variable 
quantities.  As  a  general  guide,  pitch  should  be  added  until  the 
resulting  tar  and  pitch  mixture  sets  hard  when  pressed  on  to  a 
cold  metal  surface.  The  dolomite  is  continually  mixed  until 
every  particle  is  well  covered  with  binder  and  the  entire  mixture 
is  black.  The  quantity  of  binder  should  be  only  just  sufficient 
to  make  the  mixture  adhesive  without  giving  it  a  wet-looking 
appearance. 

This  mixing  may  be  conveniently  carried  out  in  a  small 
sand-mixing  runner  mill,  which  soon  assumes  the  temperature 
of  the  dolomite  and  then  has  no  further  tendency  to  chill  and 
clog  up.  All  brick  surfaces  on  to  which  the  dolomite  is  rammed 
are  first  brushed  over  with  thin  tar  to  facilitate  adhesion.  The 
prepared  dolomite,  after  being  allowed  to  cool  down  to  between 
100°  and  150°  F.,  is  spread  in  a  layer  not  exceeding  1|  inches 
thick,  and  rammed  hard  until  it  rings  under  the  blow  of  the 
ramming  iron.  Only  a  small  area  is  covered  and  rammed  at 
one  time,  otherwise  imperfectly  rammed  portions  might  escape 
the  notice  of  the  men  and  result  in  soft  porous  patches.  A  hard 
rammed  surface,  when  once  cold,  should  be  roughed  all  over 
with  a  pick  to  ensure  a  proper  bond  with  any  fresh  mixture 
rammed  upon  it.  The  actual  bottom  is  usually  rammed  up 
flat  to  the  correct  thickness  in  the  middle,  the  sides  being  after- 
wards sloped  up  until  the  entire  hearth  has  assumed  its  proper 
shape. 

When  the  furnace  body  lining  has  been  completed,  the  roof 
is  placed  in  position,  care  being  taken,  before  finally  bolting  it 
down,  to  centre  it  to  the  electrode  holders  in  order  to  ensure 
proper  clearance  for  each  electrode. 

Basic  Linings  with  Bottom  Conductors. — Basic  lined  fur- 
naces, in  which  a  conductive  hearth  becomes  an  integral  part 
of  the  load  circuit,  are  lined  in  a  rather  similar  manner  to  that 


FUBNACE  LINING  AND  LINING  KEPAIRS  297 

just  described,  the  only  material  difference  lying  in  the  con- 
struction of  the  hearth  itself.  Since  it  is  necessary  to  distribute 
the  current  from  the  bottom  conductor  proper  to  the  conductive 
hearth  over  as  large  an  area  as  possible,  special  methods  have 
to  be  followed,  which  vary  only  in  detail  for  the  several  different 
types  of  furnaces  belonging  to  this  class.  For  those  types  of 
bottom  electrode  furnaces  in  which  metallic  bottom  electrodes 
penetrate  the  hearth  and  convey  the  current  direct  to  the 
metallic  charge  independently  of  the  hearth  itself,  no  special 
precautions  are  necessary  to  secure  definite  distribution  of 
current  through  the  hearth  itself. 

True,  conductive  hearth  furnaces,  on  the  other  hand,  present 
a  different  problem  and  demand  special  consideration.  "Black 
basic,"  consisting  of  magnesite  or  dolomite  mixed  with  a  pitch- 
tar  binder,  is  generally  used  for  forming  the  hearth  and  is  pre- 
pared and  rammed  to  shape  just  in  the  same  way  as  for  non- 
conductive  bottom  furnaces.  It  is  not  a  good  conductor  of 
electricity  when  cold,  even  after  baking,  but  when  heated  the 
conductivity  progressively  improves  with  rise  of  temperature. 
This  property  points  to  a  source  of  danger  arising  from  the 
presence  of  any  low  resistance  circuits  or,  in  other  words,  from 
unequal  distribution  of  the  current  flowing  from  the  bottom 
conductors  to  the  charge  in  the  furnace.  To  take  an  extreme 
example,  assume  that  there  is  only  one  small  bottom  electrode 
embedded  centrally  beneath  the  conductive  hearth  and  supply- 
ing current  to  it.  It  is  evident  that,  as  soon  as  the  hearth 
becomes  hot  enough  to  conduct,  the  current  will  take  the 
shortest  path  through  it  to  the  metallic  charge  and  therefore  be 
concentrated  in  a  small  zone  of  high  current  density.  The 
hearth  would  then  become  heated  locally  by  the  passage  of  the 
current  and,  becoming  more  conductive,  would  allow  a  heavier 
current  to  flow.  This  might  progressively  continue  until  the 
local  hearth  temperature  became  sufficiently  high  to  cause  dis- 
integration of  the  refractory  material.  For  this  reason  it  is  of 
great  importance  to  convey  the  current  from  the  bottom  con- 
ductors to  the  hearth  in  such  a  manner  that  the  current  density 
is  as  uniform  as  possible  at  all  points,  so  that  all  risk  of  local 
heating  and  disintegration  may  be  avoided.  It  was  at  one 
time  the  practice  to  embed  two  or  more  bottom  electrodes  in 


298 


THE   ELECTRO-METALLURGY   OF   STEEL 


the  conductive  hearth,  which  were  securely  connected  to  the 
conducting  bus  bars  or  cables.  These  bottom  electrodes  con- 
sisted in  some  cases  of  carbon  blocks  and  in  others  of  water- 
cooled  metallic  blocks.  This  method  is  open  to  objection  on 
account  of  the  local  heating  in  the  conductive  hearth,  due  to 
the  high  current  density  along  the  shortest  paths  between  the 
metallic  charge  in  the  furnace  and  the  embedded  electrodes.  In 
order  to  overcome  these  disadvantages  the  following  method  has 
been  successfully  adopted,  and  can  equally  well  be  applied  to 
any  bottom  electrode  furnace  in  which  the  entire  conductive 
hearth  acts  as  a  single  unit.  This  method  can  best  be  explained 
by  reference  to  Fig.  126.  Two  narrow  rectangular  openings  D 
are  cut  in  the  bottom  shell  plate  B,  each  opening  being  about  18 


inches  from  the  end  plates  and  extending  transversely  across 
the  furnace  to  within  a  similar  distance  from  the  back  and  front 
plates.  A  fire-brick  covering  A,  4-J  inches  thick,  is  then  laid  on 
the  bottom  plate,  leaving  openings  E  to  coincide  with  those  in 
the  bottom  plate.  A  number  of  copper  bars  F  about  5  inches 
x  |  inch,  bent  at  right  angles  at  each  end,  are  laid  on  the  brick- 
work so  that  the  short  bent  portions  pass  through  the  narrow 
openings  and  project  downwards  about  6  inches.  These  copper 
bars  are  further  connected  together  at  both  ends  by  heavy 
collector  bars  G,  which  in  their  turn  are  provided  with  terminal 
heads  for  connection  to  the  bottom  conductor  cables.  By  adopt- 
ing this  system  the  bottom  electrode  may  be  regarded  as  a  single 
copper  plate  covering  an  area  immediately  below,  but  rather 
larger  than,  the  actual  working  bottom  of  the  furnace  hearth. 


FUENACE  LINING  AND   LINING  REPAIRS  299 

Carbon  paste  H  is  then  sometimes  rammed  over  the  floor  of  the 
furnace,  filling  all  spaces  between  the  copper  strips  and  cover- 
ing them  to  a  depth  of  1  to  2  inches.  "  Black  basic  "  I  is  then 
rammed  upon  this  carbon  layer  in  the  usual  manner  and  with 
a  minimum  thickness  of  about  12  inches.  To  ensure  proper 
current  distribution  over  those  areas  underlying  the  foot  of  the 
furnace  banks,  f  inch  steel  bars  are  spaced  at  intervals  and 
connect  the  interior  of  the  hearth  to  the  carbon  floor,  the  "  black 
basic  "  being  rammed  around  them  in  the  usual  manner. 

Furnace  bottoms  constructed  in  this  way  have  given  very 
good  results,  and  have  lasted  upwards  of  nine  months  before 
being  renewed.  It  should  be  pointed  out  that,  according  to  the 
method  of  preparing  " black  basic"  as  previously  described  for 
non-conductive  hearth  furnaces,  a  minimum  quantity  of  carbon- 
aceous binder  is  used,  whereas  in  order  to  improve  the  con- 
ductivity of  the  hearth  of  bottom  conductor  furnaces,  it  is 
sometimes  advisable  to  mix  in  a  small  quantity  of  ground  carbon, 
more  especially  for  ramming  that  portion  of  the  hearth  nearest 
the  bottom  electrode;  this  practice  is  not,  however,  always 
followed.  To  improve  the  conductivity  of  the  hearth  material 
a  small  quantity  of  magnesite  and  iron  nails  are  also  sometimes 
added,  but  with  doubtful  advantage. 

Furnace  Roofs. — Furnace  roofs  are  usually  built  of  silica 
bricks  in  a  rigid  steel  framework,  the  correct  position  of  the 
electrode  openings  being  obtained  by  setting  the  bricks  on  a 
prepared  template  upon  which  the  roof  frame  is  carefully 
centred.  Provision  is  made  for  free  expansion  of  the  brickwork 
by  interposing  a  large  number  of  thin  wood  sheets,  -^  inch  to 
y1^  inch  thick,  between  the  bricks,  both  in  the  cross  and  face 
joints.  Without  taking  this  precaution  the  brickwork,  especi- 
ally in  the  case  of  large  roofs,  might  crush  and  "  spall "  badly 
during  the  preliminary  heating. 

Drying  out  and  Baking  in  Furnace  Linings. — Before  actual 
melting  operations  are  begun,  it  is  customary  to  dry  out  the 
brickwork  and  bake  in  the  basic  bottom.  This  operation  is  im- 
portant for  several  reasons  and  should  never  be  omitted. 

Silica  brickwork,  which  generally  constitutes  the  upper  wall 
and  roof  lining  of  basic  furnaces,  requires  slow  and  careful 
heating  to  remove  all  moisture,  and  to  minimise  the  unequal 


300  THE   ELECTRO-METALLTJKGY   OF    STEEL 

expansion  due  to  the  difference  in  temperature  between  the 
inner  and  outer  surfaces,  which  might  give  rise  to  "spalling". 
It  is  advisable  also  during  this  preliminary  heating  to  finish  at 
a  temperature  that  is  just  sufficient  to  glaze  the  silica  brickwork 
without  producing  any  tendency  to  drip.  During  the  first  three 
or  four  heats  the  roof  should  be  carefully  nursed  and  never 
heated  to  a  prolonged  temperature  higher  than  the  softening 
point  of  the  brickwork.  By  following  this  practice  the  roof  can 
be  well  "  seasoned  "  and  its  life  considerably  prolonged  for  the 
reasons  given  in  Chapter  XIII.  The  heating  and  baking-in  of 
the  basic  hearth  demands  equal  care,  and  is  best  carried  out 
slowly.  During  this  operation  the  volatile  constituents  of  the 
pitch-tar  binder  are  expelled,  and  if  driven  off  too  rapidly,  will 
cause  the  soft  plastic  material  to  swell  and  lose  its  solidity. 

The  preliminary  warming  of  the  furnace  is  therefore  a 
matter  that  must  be  regulated  by  the  necessarily  slow  rate  of 
drying  and  heating  the  silica  roof,  which  is  usually  raised  to  a 
very  dull  red  heat  in  about  5-8  hours  or  longer,  according  to  the 
humidity  of  the  brickwork  and  span  of  the  roof.  After  this 
point  has  been  reached,  the  temperature  may  be  slowly  raised 
until  the  interior  of  the  furnace  is  about  1100°  C.  ;  this  tem- 
perature is  then  maintained  until  the  gases  given  off  by  decom- 
position of  the  pitch-tar  binder  are  no  longer  seen  to  burn  at 
any  crevices  inside  the  furnace  formed  by  shrinkage  of  the 
dolomite  away  from  the  brickwork.  The  temperature  is  then 
further  raised  and  held  for  about  half-an-hour  in  order  to  glaze 
the  silica  brickwork  and  frit  the  surface  of  the  basic  hearth. 

This  completes  the  prepartion  of  the  furnace  lining  prior  to 
receiving  its  first  charge. 

Mode  of  Heating. — In  the  case  of  indirect  arc  furnaces  the 
heat  can  be  applied  simply  by  striking  an  arc  between  the 
electrodes,  so  that  the  entire  lining  is  heated  by  radiation  just 
as  when  melting.  This  method  is  not  applicable  to  direct  arc 
furnaces,  which  require  a  conductive  charge  to  complete  the 
circuit  of  the  current  flowing  through  each  electrode,  irrespective 
of  whether  the  path  be  between  two  or  more  vertical  upper 
electrodes,  or  from  one  or  more  upper  electrodes  through  a  con- 
ductive hearth  to  a  bottom  electrode  and  return  conducting 
cables. 


FUENACE   LINING  AND  LINING  EEPAIKS  301 

In  those  cases  where  the  circuit  is  entirely  independent  of 
the  furnace  hearth,  as  in  the  Heroult  type,  it  is  necessary  to 
provide  a  conductive  charge  on  to  which  the  direct  arcs  may 
strike.  Hard  coke  or  broken  lumps  of  amorphous  electrode 
may  be  used  for  this  purpose,  in  which  case  the  lumps  are 
charged  on  to  the  hearth  up  to  the  charging  door-sill  level. 
The  arcs  will  then  strike  on  to  the  carbon  bed,  and  the  load  be 
controlled  and  properly  balanced,  just  as  when  melting  a  charge 
of  cold  scrap.  If  the  coke  used  is  in  small  pieces,  the  resistance 
of  the  bed  when  cold  may  be  considerable,  and  it  will  then  be 
necessary  to  lower  the  electrodes  on  to  the  charge  to  effect 
passag^  of  the  current  which  generates  heat  by  resistance. 
Further,  since  the  resistance  of  the  coke  is  high  and  by  no 
means  uniform,  the  position  of  the  true  neutral  point  in  the 
case  of  three-phase  furnaces  may  vary  considerably,  and  balance 
of  current  through  each  electrode  will  not  necessarily  indicate 
uniform  heating  of  the  coke  bed.  For  this  reason  it  is  some- 
times best  to  control  the  preliminary  heating  by  actual  observa- 
tion, the  greatest  heat  being  produced  where  the  resistance  of 
the  charge  is  highest.  Thus,  by  lowering  one  electrode  on  to 
the  bed  and  raising  another,  the  zone  of  heating  may  be  trans- 
ferred as  desired.  Directly  the  coke  becomes  heated  through- 
out to  dull  redness  its  resistance  rapidly  falls  and  becomes 
uniform,  and  balance  of  current  then  correctly  indicates  uni- 
formity of  heating.  Lamps,  which  are  sometimes  connected 
between  each  electrode  and  a  common  point  in  the  furnace 
hearth  for  the  purpose  of  roughly  indicating  balance  of  arc 
voltages,  fail  to  operate  at  the  commencement  of  heating  owing 
to  the  high  and  irregular  resistance  of  the  coke  bed  and  the 
low  conductivity  of  the  cold  furnace  hearth  in  which  their 
common  point  is  embedded.  As  heating  proceeds,  the  coke  bed 
becomes  highly  conductive,  and  longer  arcs  are  then  formed 
under  each  electrode  in  place  of  the  multitudinous  small  arcs 
striking  between  the  individual  lumps  of  coke  ;  resistance  heat- 
ing at  the  same  time  gives  place  to  arc  heating.  These  free 
direct  arcs  cause  the  electrodes  to  slowly  bore  downwards  to 
the  bottom,  and,  unless  the  holes  formed  are  rilled  with  fresh 
coke,  the  close  proximity  of  the  arcs,  which  should  not  be  less 
than  9  inches  from  the  bottom,  will  cause  disintegration  of  the 


302  THE   ELECTRO-METALLURGY   OF    STEEL 

basic  material.  Such  burning  of  the  dolomite  or  magnesite 
hearth  is  generally  accompanied  by  large  volumes  of  a  heavy 
grey-black  smoke,  resulting  from  the  reduction  of  magnesium 
compounds  by  carbon. 

The  above  procedure  requires  to  be  modified  in  the  case  of 
furnaces  provided  with  conductive  hearths,  which  in  normal 
operation  constitute  a  part  of  the  load  circuit.  A  basic  hearth, 
when  cold,  is  never  sufficiently  conductive  to  perform  its  proper 
function,  and  remains  inoperative  until  thoroughly  heated.  In 
most  types  of  conductive  hearth  furnaces,  a  load  circuit  can  be 
formed  independently  of  the  hearth,  and,  although  it  is  then 
impossible  to  obtain  full  load  or  proper  conditions  of  balance, 
it  is  nevertheless  possible  to  convert  sufficient  electrical  energy 
into  heat  for  the  purpose  of  drying  out  and  baking  in  the  basic 
hearth  with  the  aid  of  a  coke  or  carbon  bed  as  above  described. 
As  the  bottom  becomes  hot,  the  basic  material  will  begin  to 
conduct  current,  and  finally  permit  the  proper  electrical  con- 
ditions of  balance  and  load  distribution  to  be  secured.  The 
passage  of  the  current  through  the  warm  unbaked  conductive 
hearth  promotes  internal  resistance  heating,  which  may  con- 
siderably influence  the  normal  rate  at  wrhich  the  volatile  con- 
stituents of  the  binder  should  be  driven  off.  For  this  reason, 
together  with  the  danger  of  internal  local  heating  previously 
mentioned,  the  load  is  kept  at  a  low  figure,  even  after  the  hearth 
has  begun  to  carry  current. 

In  certain  types  of  furnace,  notably  the  original  Stobie  two- 
phase  design,  the  hearth  is  an  integral  portion  of  the  only  pos- 
sible load  circuits.  In  this  case  the  furnace  must  be  preliminarily 
heated  by  gas  or  oil,  until  the  hearth  becomes  sufficiently  con- 
ductive to  allow  internal  electrical  methods  of  heating  to  be 
followed. 

Furnaces  in  which  metallic  conductors  are  employed  for  the 
passage  of  the  main  current  from  the  conductive  charge  to  the 
bottom  return  cables  belong  to  yet  another  class  which  requires 
special  mention.  The  Snyder  and  Girod  furnaces  are  the  only 
well-known  examples  of  this  type,  and  in  each  case  the  metallic 
electrode  or  pole  is  embedded  in  the  hearth  which  it  penetrates. 
The  simple  method  of  heating  electrically  with  a  charge  of  coke 
can  be  again  followed  for  the  baking-in  operations,  but  it  is 


FURNACE    LINING   AND   LINING   REPAIRS  303 

advisable  to  take  precautions  against  excessive  local  heating 
and  disintegration  of  the  basic  material  surrounding  the  nose 
end  of  the  metallic  conductor.  This  can  readily  be  done  by 
placing  a  few  magnesite  bricks  round  the  exposed  end  to  form 
a  short  stack  about  6  inches  high  ;  a  short  length  of  graphite 
electrode  is  then  placed  vertically  within  it  and  protrudes  2  inches 
or  3  inches  above  the  level  of  the  magnesite  bricks,  the  whole 
being  then  covered  over  with  coke  in  the  usual  way.  On 
passage  of  the  current,  the  contact  between  the  graphite  stub 
and  the  metallic  pole  piece  will  rapidly  improve  by  fusion,  until 
there  is  no  longer  serious  local  heating  at  that  point,  which  is 
level  with  the  basic  hearth.  The  current  will  then  naturally  pass 
through  the  graphite  stub  to  the  coke  in  contact  with  it,  so  that 
the  zone  of  heating  is  lifted  at  least  6  inches  above  the  bottom. 

Acid  Linings. — The  use  of  acid  linings  is  at  present  con- 
fined to  furnaces  whose  load  circuits  are  entirely  independent  of 
the  linings,  or  in  which  a  load  circuit  through  the  hearth  is  com- 
posed of  a  metallic  conductor  acting  quite  independently  of  it. 

The  only  material  difference  in  the  construction  of  acid  and 
basic  linings  lies  in  the  substitution  of  silica  brick  for  magnesite 
brick,  and  of  rammed  ganister  in  place  of  dolomite.  No  further 
mention  need  be  made  as  regards  the  brick  lining,  beyond 
pointing  out  that  it  is  unnecessary  to  dry  out  the  brickwork 
thoroughly  before  ramming  in  the  hearth  material.  The  work- 
ing hearth  is  generally  composed  of  ganister,  which  is  mixed 
with  a  pitch-tar  binder  and  rammed  to  shape.  A  mixture  con- 
sisting of  75  per  cent,  calcined  crucible  ganister,  25  per  cent, 
crushed  quartz  pebbles,  and  just  enough  binder  to  facilitate 
ramming,  gives  excellent  results  and  is  used  in  the  same  way 
as  "black  basic".  The  ganister  should  be  calcined  at  about 
1000°  C.  to  expel  water  and  volatile  matter,  which  together 
amount  to  about  2  per  cent.  The  proportions  of  crushed  calcined 
ganister  and  broken  quartz  rock  should  be  such  as  will  form  a 
compact  mass  in  which  all  the  coarser  pebbles  are  firmly  held 
in  a  solid  matrix.  Should  metallic  conductors  penetrate  the 
hearth,  the  ganister  is  rammed  round  them  without  taking  any 
special  precautions.  The  preliminary  drying  out  and  heating 
is  best  conducted  in  the  same  way  and  with  the  same  care  as 
for  basic  linings. 


304  THE    ELECTRO-METALLURGY   OF    STEEL 

Furnace  Lining  Repairs. — The  life  of  a  furnace  wall  lining  is 
usually  determined  by  the  life  of  the  roof,  which  suffers  most 
and  cannot  be  temporarily  patched  in  the  same  way  as  the  door 
jambs,  walls  and  hearth.  A  roof  may  sometimes  fail  prematurely 
owing  to  the  use  of  inferior  material,  faulty  construction,  or 
improper  treatment  during  the  initial  heating  ;  in  such  cases 
the  furnace  walls  may  be  hardly  worn,  and  the  roof  can  be 
removed  without  causing  any  damage  to  the  body  lining. 

In  normal  circumstances  a  roof  by  the  time  it  requires  to  be 
renewed  will  have  become  firmly  fritted  to  the  upper  courses  of 
the  silica  brick  walls,  and  therefore  cannot  be  detached  without 
seriously  damaging  them,  which  usually  necessitates  a  complete 
relining  of  the  walls.  A  short  life  of  a  roof,  therefore,  results  in 
unnecessary  waste  of  refractory  material,  and  so  influences  to  a 
great  extent  the  repair  costs  per  ton  of  steel.  Under  satisfactory 
working  conditions  the  walls  and  hearth  should  require  renewal 
or  repair  at  the  same  time  as  the  roof,  so  that  the  maximum 
amount  of  wear  will  be  obtained  from  all  the  refractories  used. 

The  methods  of  relining  basic  and  acid  furnaces  are  very 
similar,  and  can  be  dealt  with  jointly. 

After  pouring  the  last  heat  of  a  campaign,  the  electrodes  and 
coolers  are  dismantled  preparatory  to  removing  the  roof,  which 
is  best  done  while  the  furnace  is  still  red  hot.  This  is  some- 
times followed  by  immediate  demolition  of  the  silica  walls, 
which  part  readily  from  the  top  course  of  magnesite  bricks,  the 
brickwork  being  then  withdrawn  with  the  aid  of  long  hooks. 
The  above  practice  is,  of  course,  not  always  followed  when  the 
duration  of  the  repair  is  not  of  great  importance.  After 
demolition  and  removal  of  the  silica  brick,  the  furnace  is  allowed 
to  cool  off  sufficiently  for  work  to  be  proceeded  with  from  within 
the  furnace  body.  The  courses  of  magnesite  brick  that  have 
been  badly  eroded  and  require  renewal  are  then  removed,  to- 
gether with  the  entire  semi-fused  slaggy  covering  of  the  dolomite 
hearth.  Particular  attention  is  paid  to  this,  and  at  the  same 
time  care  is  taken  to  remove  all  trace  of  steel  remaining  on  the 
bottom  or  absorbed  by  the  hearth  material  in  porous  places. 
All  this  loose  material  is  then  thrown  on  one  side,  and  the  brick- 
work and  bottom  brushed  over  with  a  stiff  brush  to  remove  all 
powder  and  loose  material.  This  will  enable  better  examina- 


FURNACE   LINING  AND   LINING  EEPAIRS  305 

tion  of  the  bottom  to  be  made.  The  hearth  material  should, 
after  brushing  over,  appear  quite  black  and  solid.  Black  basic 
or  ganister  should  never  be  rammed  on  to  steel  or  slag,  which, 
on  subsequent  fusion,  may  cause  the  newly  rammed  portion  to 
become  detached ;  for  this  reason  it  should  only  be  rammed  on 
a  solid  foundation,  which  necessitates  careful  removal*  of  all 
friable  portions  of  the  old  hearth. 

After  the  above  preparation  of  the  bottom  has  been  com- 
pleted, it  is  usual  to  rebuild  the  entire  walls.  The  hearth,  after 
being  thoroughly  cleaned,  is  then  brushed  over  with  a  very  small 
quantity  of  thin  tar  to  effect  adhesion  of  the  fresh  material, 
which  is  then  rammed  up  in  the  customary  manner.  After 
completion  of  the  latter  operation  the  furnace  body  is  ready  to 
receive  a  new  roof,  which  is  carefully  centred  to  the  electrodes 
in  cases  where  the  latter  pass  through  it.  Kefixing  of  the  sundry 
accessories,  such  as  coolers,  water  pipes,  etc.,  concludes,  the 
furnace  repair.  The  roof  and  furnace  walls  require  the  same 
careful  heating  up  as  previously  described,  but  the  time  required 
for  baking-in  the  newly  rammed  hearth  may  be  reduced  accord- 
ing to  the  quantity  and  depth  of  the  fresh  material  rammed. 

Life  of  Furnace  Lining. — The  various  chemical  and  physical 
conditions  which  so  greatly  influence  the  life  of  a  refractory 
lining  have  been  fully  dealt  with  in  Chapter  XT.  These  con- 
ditions are  not  only  inherent  to  the  metallurgical  process  per- 
formed, but  depend  very  largely  upon  the  electrical  and 
constructional  design  of  the  furnace  employed.  For  these 
reasons,  the  life  of  a  furnace  lining  is  most  variable  and  only 
a  general  idea  can  possibly  be  given. 

The  basic  lining  of  a  3  to  6-ton  polyphase  direct  arc  furnace, 
working  with  a  low  arc  voltage  and  in  operation  day  and  night, 
should  last  four  to  five  weeks  before  requiring  renewal  of 
the  upper  walls  and  roof.  The  hearth  will,  of  course,  also  re- 
quire considerable  repairing  but  not  entire  renewal ;  this  applies 
to  furnaces  melting  and  refining  cold  scrap.  In  the  case  of 
furnaces  being  used  for  only  eight  to  twelve  hours  a  day,  the 
life  of  the  lining  will  be  certainly  prolonged,  but  not  in  propor- 
tion to  the  diminished  output ;  the  cost  of  repairs  per  ton  of 
steel  will  in  consequence  be  heavier,  representing  an  increase 
of  about  10  to  20  per  cent. 

20 


CHAPTEE  XV. 

PKOPEKTIES  AND  MANUFACTURE  OF  CARBON  ELECTRODES. 

IN  all  electro-chemical  and  electro-metallurgical  work  dependent 
upon  electrolytic  or  electro-thermal  action,  the  term  "  electrode  " 
is  applied  to  any  terminal  conductor  which  conveys  electrical 
energy  into  an  apparatus  in  a  manner  consistent  with  a  par- 
ticular mode  of  current  distribution.  The  material  under  treat- 
ment may  be  solid,  liquid,  or  gaseous.  In  all  metallurgical  arc 
furnaces  there  are  at  least  two  electrodes,  one  of  which  must  be 
adjustable  and  electrically  insulated  from  the  furnace  body  and 
earth.  For  this  purpose  a  movable  electrode  is  always  used  as 
one  terminal  of  the  arc,  and  is,  therefore,  exposed  locally  to  the 
highest  arc  temperatures. 

As  regards  arc  furnaces,  the  materials  used  for  electrode 
manufacture  must  either  resist  physical  change  at  arc  tempera- 
ture, or  must  be  water-cooled  to  prevent  destruction.  In  the 
latter  case,  the  loss  of  heat  is  far  too  great  for  the  economical 
operation  of  metallurgical  furnaces,  so  that  carbon  can  alone  be 
used  for  electrodes  other  than  those  which  are  not  under  the 
direct  influence  of  the  arc.  Carbon  exists  in  both  amorphous 
and  crystalline  states,  which  have  very  distinct  physical 
properties. 

Amorphous  Carbon. — There  are  numerous  varieties  of 
amorphous  carbon,  both  natural  and  artificial,  of  which  those 
usually  employed  in  electrode  manufacture  are  : — anthracite, 
gas  retort  carbon,  pitch  coke  and  petroleum  coke.  Their  density, 
porosity,  and  electrical  conductivity  vary,  and  the  properties  of 
an  electrode  made  from  these  materials  will  depend  largely 
upon  the  proportions  in  which  they  are  present  and  their 
physical  condition  when  used. 

Anthracite. — High  grade  anthracite  selected  for  -electrode 
manufacture  contains  over  90  per  cent,  fixed  carbon,  the  re- 

(306) 


PBOPEETIES   AND   MANUFACTURE    OF   CAEBON   ELECTRODES      307 

mainder  being  volatile  matted  and  ash.     Below  are  given  the 
analyses  of  a  suitable  grade  before  and  after  calcination. 

Before  Calcination.  After  Calcination. 

Fixed  carbon 91-8  95-81 

Volatile  matter       ....       5-3  1-4 

SandP -8  -89 

Ash 1-75  2-0 

Anthracite  is  hard,  brittle  and  shiny,  and  leaves  no  mark 
when  rubbed  on  paper. 

Gas  Retort  Carbon. — This  form  of  carbon  is  a  product  of 
coal  distillation,  and  is  deposited  on  the  lining  of  gas-works 
retorts  as  a  hard  crust  up  to  2|  inches  thick.  It  is  finely  porous, 
hard,  varies  m  degree  of  density,  and  has  a  high  electrical  con- 
ductivity. It  is  usually  crushed  and  ground  in  mills,  and, 
owing  to  its  great  toughness,  the  small  angular  fragments  are 
liable  to  become  rounded.  A  typical  analysis  of  a  dried  sample 
is  as  follows  : — 

Fixed  carbon  .        .        .        .-.._.        .        .  94-54 

Volatile  matter -76 

S  and  P 1-3 

Ash         .        . 3-42 

Petroleum  Coke. — This  material  is  the  residue  left  after  the 
complete  destructive  distillation  of  crude  mineral  oils.  It  is 
light  and  porous,  and  is  nearly  pure  carbon  containing  a  small 
percentage  of  volatile  matter.  It  is  readily  converted  into 
graphite,  and  is  therefore  used  for  the  manufacture  of  electrodes 
suitable  for  graphitising.  Owing  to  its  freedom  from  mineral 
impurities  it  is  also  extensively  used  in  the  manufacture  of 
electrodes  for  aluminium  reduction. 

Pitch  Coke. — This  is  the  final  retort  residue  resulting  from 
the  destructive  distillation  of  coal  tar.  It  resembles  petroleum 
coke  in  character,  and  usually  contains  sufficient  volatile  matter 
to  render  a  preliminary  calcination  necessary. 

Crystalline  Carbon. — -Natural  Graphite. — This  is  a  crystal- 
line form  of  carbon  which  is  better  known  as  "  plumbago  ".  It  is 
usually  contaminated  with  mineral  matter,  which  in  the  lower 
grades  rises  up  to  30  per  cent.  The  finest  Ceylon  flake  graphite 
contains  as  much  as  99  8  per  cent,  carbon.  Natural  graphite  is 
very  soft  and  is  a  good  conductor  of  both  heat  and  electricity. 


308  THE    ELECTKOMETALLUKGY   OF    STEEL 

Artificial  Graphite. — Artificial  graphite  was  discovered  in 
1896  by  E.  G.  Acheson,  who  found  that  amorphous  carbon 
could  be  converted  into  graphite  under  the  influence  of  prolonged 
heating  at  temperatures  exceeding  2000°  C.  This  process  was 
at  once  developed  for  graphitising  moulded  articles  of  amorphous 
carbon.  Acheson  considered  that  the  presence  of  small  quantities 
of  mineral  matter  was  beneficial  rather  than  injurious  to  the 
promotion  of  the  physical  change,  but  recent  investigations  have 
shown  that  the  transformation  is  dependent  upon  the  variety  of 
amorphous  carbon  used  rather  than  upon  the  impurities  present. 
An  article  of  high  degree  of  purity,  containing  99*5  per  cent, 
carbon  can  be  easily  made  by  this  method  from  the  less  pure 
amorphous  varieties,  the  impurities  being  volatilised  during  the 
process  of  conversion. 

Artificially  graphitised  articles  are  very  soft  and  can  be  easily 
machined  to  exact  sizes.  The  electrical  conductivity  is  about 
four  times  that  of  amorphous  carbon  when  cold,  and  increases 
but  slightly  at  high  temperatures. 

Arsem  defines  graphite,  both  natural  and  artificial,  as  an 
allotropic  form  of  carbon,  having  a  specific  gravity  of  2*25  to 
2-26. 

Amorphous  Carbon  Electrodes.  —  Preparation  of  Eaw 
Materials. — The  several  varieties  of  amorphous  carbon  pre- 
viously mentioned  have  all  been  used  for  the  manufacture  of 
electrodes,  although  the  present  day  tendency  is  to  use  a  pre- 
ponderating quantity  of  anthracite,  and  in  some  cases,  to  elimi- 
nate pitch  coke  entirely.  Whatever  mixture  may  be  chosen, 
the  preliminary  treatment  before  mixing  with  the  binder  applies 
equally  in  all  cases.  The  volatile  matter  present  in  the  solid 
constituents  of  amorphous  electrode  mixtures  should  not  exceed 
about  I'O  per  cent.,  so  that  a  preliminary  calcination  at  high 
temperatures  is  always  essential. 

The  grain  size  of  the  crushed  materials  is  also  of  great  im- 
portance and  is  carefully  controlled  ;  it  is,  however,  largely  de- 
pendent upon  the  character  of  the  materials  used.  Retort 
carbon,  owing  to  its  extreme  hardness,  has  a  tendency  to 
pulverise  by  abrasion,  the  larger  grains  becoming  rounded  as  a 
result ;  this  is  a  disadvantage,  as  the  cohesive  power  is  less  than 
for  sharp  angular  particles.  Pitch  and  petroleum  cokes  are 


PKOPEETIES  AND   MANUFACTUBE   OF  CABBON   ELECTBODES      309 

friable  and,  owing  to  their  extreme  sponginess,  require  to  be 
broken  down  to  a  fine  state  of  division.  Anthracite  can  be 
easily  crushed  to  a  correct  and  uniform  size  without  making 
too  large  a  proportion  of  fines,  and,  for  this  reason  alone,  is 
superior  to  the  other  materials.  It  is  usually  calcined  in  specially 
designed  cupolas,  where  by  partial  combustion  it  provides  the 
necessary  heat  for  the  elimination  of  volatile  matter.  In 
certain  localities,  where  electric  power  is  cheap,  the  calcination 
is  performed  in  electric  shaft  furnaces,  designed  so  as  to  make 
the  process  continuous,  as  in  cupola  practice. 

Binders. — Straight-run  gas-works  pitch  is  generally  used 
for  binding  together  the  various  carbon  materials  in  the  press- 
ing operations.  It  is  kept  at  a  fixed  standard  of  viscosity  and 
softening  point  by  constant  examination,  and  the  addition  of 
hard  pitch  or  tar. 

Outline  of  Manufacture. — The  prepared  and  preheated 
carbon  materials  are  mixed  in  pan  mills  in  the  correct  proportion, 
with  about  11  per  cent,  of  pitch  binder,  drawn  direct  from  a 
special  boiler.  The  mills  are  heated  to  prevent  any  chilling  of 
the  paste  and  to  maintain  the  desired  degree  of  plasticity  up  to 
the  moment  of  passing  to  the  moulds.  The  so-called  "  paste  " 
is  slowly  transferred  to  heated  moulds,  where  it  receives  a  pre- 
liminary ramming  under  a  mechanical  ramming  head.  This 
enables  a  sufficient  quantity  of  paste  to  be  charged  into  the 
mould  for  a  given  length  of  electrode,  and  avoids  air  cavities. 
The  mould  is  then  placed  under  a  press  head  and  the  paste 
subjected  to  a  pressure  of  about  1  to  1^  tons  per  square  inch, 
which  is  maintained  for  a  few  minutes.  After  stripping  the 
moulds,  the  "green"  electrodes  are  placed  vertically  in  the 
stalls  of  a  baking  stove  and  packed  tightly  with  sand  after 
bricking  up  the  front  wall  of  each  stall.  This  packing  prevents 
oxidation  and  gives  support  to  the  tender  electrodes  while  the 
first  portion  of  the  volatile  matter  is  being  driven  off.  Extrusion 
presses  have  been  recently  applied  with  satisfactory  results  in 
the  manufacture  of  even  the  largest  sizes  of  amorphous  elec- 
trodes. Electrodes  formed  in  this  way  have,  of  course,  to  be 
machined  and  bored  afterwards  at  both  ends  for  the  screwed 
sockets. 

The  baking  operation  needs  considerable  care  and  is  controlled 


310  THE    ELECTEO-METALLUEGY   OF    STEEL 

by  recording  pyrometers.  The  heat  is  applied  very  slowly  to 
prevent  too  rapid  evolution  of  volatile  matter,  which  may  either 
burst  or  distort  the  electrodes.  The  full  furnace  temperature 
of  1200°  C.,  which  is  only  reached  after  seven  days'  firing,  is 
kept  fairly  constant  for  a  day  or  two,  after  which  the  fires  are 
drawn.  It  usually  takes  about  ten  days  for  the  kiln  to  cool  off 
sufficiently  for  the  removal  of  the  baked  electrodes,  and  should 
the  sand  packing  be  drawn  prematurely,  there  is  risk  of  the 
electrodes  burning.  This  duration  and  method  of  cooling  refers 
to  the  above-mentioned  type  of  furnace,  comprised  of  several 
stalls  heated  by  flues  built  in  the  dividing  walls,  and  each  con- 
taining a  large  number  of  electrodes.  In  America  the  "  green  " 
electrodes  are  packed  with  sand  or  fine  coke  breeze  in  separate 
mild  steel  boxes,  which  can  be  withdrawn  from  the  stoves  while 
still  hot,  without  any  risk  of  burning  the  electrodes.  This  latter 
method  of  charging  the  "  green  "  electrodes  is  to  be  preferred, 
as  there  is  far  more  opportunity  for  the  full  furnace  temperature 
to  penetrate  to  the  heart  of  each  electrode,  and  thus  ensure 
more  perfect  removal  of  volatile  matter.  The  cost  of  the 
mild  steel  boxes,  however,  renders  their  use  prohibitive  in  many 
cases. 

The  baked  electrodes  are  cleaned,  and  their  end  faces  and 
screw  threads  trimmed  if  necessary.  An  electrical  conductivity 
test  is  also  applied  before  dispatching. 

The  screw  nipples  are  made  in  substantially  the  same 
manner,  being  pressed  in  split  moulds  and  at  a  slightly  different 
pressure. 

Defects  of  Amorphous  Electrodes. — Fracture. — The  most* 
serious  defect  of  amorphous  carbon  electrodes  is  their  frequent 
inability  to  withstand  sudden  subjection  to  high  temperatures 
without  cracking  and  subsequent  fracture.  In  the  case  of  arc 
furnaces  using  vertically  suspended  amorphous  electrodes,  there 
is  always  some  occasion  when  a  new  electrode  is  exposed  to  the 
interior  of  a  hot  furnace.  Should  failure  result  the  defective 
electrode  is  replaced  by  another,  which  is  then  exposed  to  the 
same  conditions  and  may  probably  behave  in  like  manner.  When 
electrodes  fail  in  this  way,  even  after  slow  heating  to  a  full 
furnace  temperature,  no  preliminary  precaution  will  suffice  to 
remedy  the  inherent  defect. 


PROPERTIES   AND   MANUFACTURE   OF   CARBON   ELECTRODES      311 

Fractures  of  pressed  electrodes  occur  along  transverse  or 
inclined  irregular  planes,  which  do  not  necessarily  mark  the 
junction  of  zones  of  unequal  temperature.  This  fact  would 
seem  to  disprove  any  theory  based  upon  the  setting  up  of  un- 
equal mechanical  stresses  by  irregular  heating  of  different  trans- 
verse sections,  which,  as  in  the  analogous  case  of  a  silica  brick, 
results  in  fracture  or  "  spalling  ".  Experience  also  shows  that 
fracture  only  occurs  after  the  electrode  has  attained  a  bright 
yellow  heat,  although  a  skin  crack  may  be  sometimes  seen  at 
lower  temperatures.  It  is  probable,  therefore,  that  internal 
forces  are  set  up  at  a  temperature  which  is  certainly  higher  than 
that  at  which  mechanical  stresses,  due  to  unequal  heating  of  the 
skin  relative  to  the  centre,  are  likely  to  cause  fracture.  Unequal 
and  sudden  expansion  in  the  case  of  very  dense  and  fine-grained 
electrodes  may  cause  fracture,  but  it  is  undoubtedly  the  presence 
of  volatile  matter  in  the  baked  electrode  that  is  the  primary 
cause  of  failure. 

Screw  joints  are  frequently  a  marked  source  of  weakness, 
and  sometimes  break  without  any  blow  being  given  to  the 
electrode.  This  type  of  screw  failure  usually  occurs  when  the 
joint  is  only  dull  red  or  even  black  hot,  and  very  seldom  when 
it  shows  considerable  resistance  heating.  Failure  in  this  case 
is  probably  due  to  unequal  expansion  of  the  screw  and  the 
electrode  sockets,  especially  when  the  joint  has  been  too  tight. 

Disintegration. — The  resistance  of  the  skin  of  an  electrode 
to  combustion  and  disintegration  has  an  important  influence  on 
its  life.  Combustion  of  the  smooth  skin  usually  commences  at 
a  dull  red  heat  and,  when  once  started,  promotes  more  rapid 
disintegration  of  the  matrix,  which  leaves  the  coarser  grains 
standing  out  and  exposes  a  larger  surface  to  oxidation.  Com- 
bustion of  the  coarser  grains  proceeds  slowly  and  disintegration 
might  be  arrested,  were  it  not  that  these  grains  are  easily  de- 
tached and  a  fresh  surface  then  exposed  to  oxidation. 

Local  Heating  at  Joints. — Amorphous  electrodes,  when 
joined  together  by  a  screw  nipple,  frequently  show  local  resist- 
ance heating,  extending  over  a  region  5  or  6  inches  above  and 
below  the  joint.  When  the  joint  is  raised  above  the  combustion 
temperature  of  the  carbon,  disintegration  follows,  which  may 
cause  very  considerable  "necking".  This,  however,  is  seldom 


312  THE   ELECTRO-METALLURGY   OF    STEEL 

accompanied  or  followed  by  the  fracture  of  a  nipple,  but  results 
in  undue  electrode  consumption.  The  screw  socket  walls  may 
become  thinned  by  disintegration  to  such  extent  that  they  are 
no  longer  strong  enough  to  support  the  weight  of  a  short  stub 
end,  which  would  otherwise  be  consumed  before  such  a  con- 
dition of  the  socket  walls  was  reached ;  for  this  reason  short 
stub  ends  will  be  lost,  and,  if  not  purposely  removed  beforehand, 
may  fall  into  a  bath  of  steel  and  cause  considerable  metallurgical 
difficulties  and  delays. 

Surface  Irregularities. — The  larger  sizes  of  amorphous 
electrodes  after  removal  from  the  moulds  are  packed  vertically 
in  the  baking  furnaces  and,  since  they  support  their  own  weight, 
the  bottom  end  will  accommodate  itself  to  any  unevenness  of 
the  supporting  tile  or  floor  during  the  process  of  baking.  The 
bottom  may  easily  be  distorted  from  a  plane  at  right  angles  to 
the  axis  of  the  screw  socket ;  this  is  a  serious  matter  in  joining, 
and  may  often  lead  to  broken  nipples,  owing  to  the  side  strain 
exerted  upon  them  when  screwing  the  electrodes  together.  The 
nipple  will  not  necessarily  break  when  cold,  but  may  do  so  later 
in  service,  if  a  small  crack  or  fault  has  been  once  developed 
within  it  at  the  time  of  joining. 

Surface  irregularities  in  the  screw  socket  are  also  trouble- 
some and  are  generally  removed  at  the  electrode  factory  before 
dispatch. 

Defects  in  the  form  of  surface  deformations  are  occasionally 
found,  and  may  give  rise  to  considerable  trouble  when  lowering 
the  electrode  through  its  holder,  and  by  causing  an  uneven  con- 
tact surface. 

Mechanical  Disintegration  by  the  Arc. — This  defect  is  not 
common,  but  when  apparent  sometimes  causes  metallurgical 
difficulties.  The  coarser  particles  of  the  electrodes  are  mechani- 
cally detached  from  a  loose  matrix  by  the  disruptive  action  of 
the  arc ;  the  ejected  particles  float  on  the  slag  and  reduce  its 
powers  of  oxidation. 

Chemical  and  Physical  Characteristics. — Volatile  Matter.— 
Volatile  matter  is  responsible  for  the  majority  of  failures  by 
fracture.  The  manner  in  which  faulty  electrodes  break,  up  has 
been  already  described,  and  it  was  explained  that  fracture  could 
hardly  be  accounted  for  by  the  unequal  expansion  of  the  skin  *.* 


PROPERTIES   AND   MANUFACTURE    OF   CARBON   ELECTRODES      313 

relatively  to  the  core.  In  an  investigation  to  determine  the  cause 
of  bursting,  sufficient  evidence  was  obtained  from  the  behaviour 
of  both  good  and  bad  electrodes  to  attribute  definitely  the 
cause  of  failure  to  excessive  quantities  of  volatile  matter.  The 
determination  of  volatile  matter  in  baked  electrodes  and  raw 
carbon  materials  by  the  standard  chemical  method  is  more  com- 
parative than  absolute,  and  the  results  obtained  cannot  be 
accepted  with  confidence.  A  modification  of  the  standard 
method,  embodying  the  use  of  vacuum  furnaces,  may  yield 
reliable  results,  but  is  too  elaborate  for  ordinary  works  practice. 
Reliable  results  may,  however,  be  obtained  by  a  crude  but  suffi- 
ciently accurate  method,  which  merely  consists  of  heating  a 
large  crucible  containing  1  kg.  of  the  finely  powdered  dry 
material  in  a  small  assay  wind  furnace  for  2  to  3  hours.  A  lid 
is  carefully  luted  on  beforehand,  and  is  not  removed  until  the 
crucible  has  become  cold.  The  powdered  material  is  then 
transferred  to  a  physical  balance  and  weighed.  Any  error 
through  loss  in  transfer  is  insignificant,  and  reheating  in  the 
furnace  will  not  produce  any  further  loss  of  weight.  The  tem- 
perature of  the  crucible  should  be  at  least  1200°  C.,  and  is 
therefore  more  comparable  with  working  conditions. 

It  is  to  be  naturally  expected  that  the  rapid  heating  of 
an  electrode  will  cause  sudden  volatilisation  of  any  volatile 
matter  at  a  high  temperature,  which,  moreover,  cannot  readily 
escape  if  the  electrode  is  very  dense  and  compact.  The  gas 
then  exerts  high  internal  pressures,  which  will  eventually  cause 
fracture  along  any  plane  of  weakness.  Faulty  electrodes,  when 
used  in  open  top  ferro-alloy  furnaces,  burst  off  in  slices,  which 
have  become  heated  to  a  temperature  well  above  that  to  which 
the  electrodes  were  exposed  during  the  process  of  baking  ;  small 
flames  may  also  be  seen  issuing  from  cracks  in  cases  where 
actual  rupture  has  not  occurred.  The  volatile  matter  should 
not  exceed  1*75  per  cent,  in  a  baked  electrode,  and  should  be 
below  this  figure  if  the  body  of  the  electrode  is  very  dense  and 
compact. 

Grain  Size  and  Porosity. — The  crushing  and  grinding  of  the 
raw  materials  before  mixing  is  always  carefully  controlled  to 
ensure  uniformity  of  the  electrode  structure.  The  difficulty  of 
preventing  pitch  or  petroleum  coke  being  over-pulverised  during 


314  THE    ELECTRO-METALLURGY   OF    STEEL 

the  grinding  operation  has  already  been  mentioned,  but  this 
difficulty  does  not  occur  to  such  an  extent  with  anthracite, 
owing  to  its  toughness  and  tendency  to  break  along  definite 
cleavage  planes.  The  ideal  sizing  analysis  for  a  very  dense 
electrode  would  be  such  that  the  interstices  between  the  largest 
particles  contiguous  with  one  another  would  be  filled  with  the 
minimum  number  of  particles  successively  of  smaller  size,  so 
that  the  amount  of  dust  required  to  fill  the  very  smallest  inter- 
stices would  be  reduced  to  a  minimum.  It  is,  of  course,  im- 
possible to  achieve  such  conditions  in  practice,  and,  in  fact,  it  is 
better  to  avoid  making  a  proportion  of  fines  which  would  satisfy 
such  a  case.  A  certain  degree  of  porosity  is  necessary,  as  it 
undoubtedly  assists  the  escape  of  volatile  gases  during  baking 
operations  and  reduces  the  possibility  of  fracture  in  service  by 
releasing  any  pressure  exerted  by  a  rapid  internal  generation  of 
gas. 

The  porosity  of  a  baked  electrode  is  calculated  from  the 
actual  and  apparent  specific  gravity  figures  and  is  usually  about 
25  per  cent.  The  grading  analysis  of  the  dry  crushed  material 
for  this  degree  of  porosity  should  not  show  more  than  20  per 
cent,  capable  of  passing  a  40  mesh  sieve.  The  above  figures,  in 
both  cases,  refer  to  electrodes  that  are  composed  entirely  of 
anthracite  mixed  with  only  11-12  per  cent,  of  binder.  The 
degree  of  porosity  is  limited  by  the  increase  of  electrical  re- 
sistance and  the  greater  tendency  of  the  electrode  to  burn  and 
disintegrate,  and  should  be  only  high  enough  to  permit  the  free 
escape  of  volatile  matter  during  the  baking  process  and  to 
minimise  risk  of  failure  by  further  expulsion  of  volatile  matter 
in  service. 

Ash. — The  percentage  of  ash  is  not  of  primary  importance, 
as  within  the  normal  degree  of  variation  it  in  no  way  affects  the 
mechanical  strength  of  electrodes  and  does  not  make  any  ap- 
preciable difference  in  electrode  consumption.  English  anthra- 
cite electrodes  generally  contain  less  ash  than  American,  owing 
to  the  greater  purity  of  the  anthracite  used,  and  will  usually 
show  about  2*5  per  cent,  on  analysis. 

Electrical  Conductivity. — The  electrical  conductivity  of  an 
amorphous  electrode  is  not  in  itself  a  matter  of  great  importance, 
since  the  heat  generated  by  resistance  in  an  electrode  of  proper 


PKOPEKTIES  AND   MANUFACTUKE   OF   CARBON   ELECTRODES      315 

dimensions  is  small  compared  with  the  heat  conducted  through 
it  from  the  interior  of  a  hot  furnace.  Assuming  there  is  no 
internal  resistance  heating,  then  the  temperature  of  the  electrode 
at  a  given  distance  above  the  roof  will  depend  upon  the  thermal 
conductivity  of  the  carbon  material,  and  the  rate  at  which  heat 
is  absorbed  by  the  portion  inside  the  furnace  and  radiated  to 
the  atmosphere  from  the  exposed  portion  above  the  furnace  roof. 
On  the  other  hand,  if  the  normal  resistance  heating  due  to  the 
passage  of  a  heavy  current  were  the  only  source  of  heat  in- 
fluencing the  temperature  of  an  electrode  outside  the  furnace, 
it  would  be  found  that  the  actual  temperature  rise  at  various 
points  is  far  lower  than  it  is  in  actual  practice,  where  the  effect 
of  thermal  conductivity  outweighs  that  of  electrical  resistance 
heating.  This  argument,  it  must  be  understood,  does  not 
apply  to  electrodes  when  deliberately  overloaded,  and  to  screw 
joints  which  often  exhibit  local  heating. 

An  electrical  conductivity  test  is  made  not  so  much  for  the 
simple  determination  of  specific  resistance  as  for  the  purpose  of 
indicating  whether  the  baking  operation  has  been  successfully 
carried  out.  The  variation  of  the  resistance  is  never  great  in 
electrodes  made  of  the  same  materials  and  under  the  same 
manufacturing  conditions,  but  any  small  difference  that  may  be 
found  is  considered  to  be  due  to  the  varying  extent  by  which 
the  carbon  grains  are  bound  together.  If  the  baking  is  not 
carried  sufficiently  far  the  volatile  constituents  of  the  binder 
are  improperly  removed,  and  the  added  mechanical  adhesion  of 
the  particles  due  to  the  final  coking  of  the  non-volatile  residue 
of  the  binder  is  not  attained.  It  has  been  explained  how  all 
serious  failures  of  electrodes  are  primarily  due  to  the  presence 
of  volatile  matter,  so  that  a  rapid  test  which  will  give  an  indica- 
tion of  their  behaviour  is  of  great  value.  In  some  electrode 
factories  all  electrodes  that  fail  to  come  up  to  a  definite  standard 
of  conductivity  are  rejected  and  returned  to  the  baking  stoves. 
The  specific  resistance  is  about  *00124  ohm  per  inch  cube,  or 
•00315  ohm  per  centimetre  cube. 

The  apparatus  used  for  conducting  the  resistance  test  is 
shown  diagramatically  in  Fig.  127.  The  method  is  based  on 
the  voltage  drop  between  certain  fixed  points  along  an  electrode 
when  a  known  current  is  passing  through  it.  To  save  constant 


316  THE    ELECTEOMETALLUEGY   OF    STEEL 

calculation  the  current  is  adjusted  to  definite  values  by  means 
of  a  variable  resistance,  and  the  distance  along  which  the 
voltage  fall  is  measured  is  also  kept  at  a  definite  figure,  accord- 
ing to  the  diameter  of  the  electrode.  A  "  constant "  for  each  set 
of  conditions  is  usually  worked  out,  and  this  figure,  multiplied 
by  the  millivoltmeter  reading,  gives  at  once  the  resistance  either 
in  ohms  per  inch  or  centimetre  cube. 

Graphite  Electrodes. — Manufacture. — The  electro-thermic 
process  for  converting  the  several  forms  of  amorphous  carbon 
into  graphite  has  already  been  mentioned,  but  it  might  be 
added  that  anthracite  is  difficult  to  convert,  whereas  petroleum 

or  pitch  coke  is  more  easily  trans- 
formed than  any  other  variety  of 
carbon. 

The  electro-thermic  process  for 
converting  amorphous  carbon  into 
graphite  has  been  generally  used  for 
the  manufacture  of  graphite  elec- 
trodes. Kecent  attempts,  however, 
have  been  made  to  use  natural  gra- 

LtE! VWWV  phite,  and  for  this  purpose  a  special 

FIG.  127?*"  method   of   purifying    crude    impure 

plumbago    is    now   being    developed 

commercially  in  Italy.  Artificial  graphite  electrodes  are  made 
by  subjecting  amorphous  carbon  electrodes  to  a  prolonged  heat- 
ing at  a  very  high  temperature  in  specially  designed  resistance 
furnaces. 

The  amorphous  carbon  electrodes,  consisting  almost  entirely 
of  petroleum  coke,  are  formed  by  extrusion,  cut  off  to  length, 
and  undergo  the  usual  baking  treatment.  These  electrodes  are 
then  carefully  packed  in  coke  breeze  on  the  furnace  hearth  to 
form  a  rectangular  pile  between  heavy  carbon  terminal  blocks. 
The  whole  is  then  covered  with  more  breeze  and,  finally,  a  re- 
fractory covering.  A  current  of  very  high  density  is  passed 
through  the  charge  from  end  to  end,  and  the  voltage  varies  as 
conversion  proceeds.  Heat  is  generated  internally  by  electrical 
resistance  which  enables  the  critical  temperature  of  conversion, 
said  to  be  about  2000°  C.,  to  be  easily  reached.  The  resistance 
of  the  entire  charge  falls  until  conversion  to  graphite  is  com- 


PBOPEETIES   AND   MANTJFACTUEE   OP   CAEBON   ELECTEODES      317 

plete,  which  serves  as  the  only  indication  of  the  progressive 
change  taking  place.  The  furnace  takes  several  days  to  cool 
off  sufficiently  for  demolition.  The  graphitised  electrodes  are 
carefully  cleaned  and  sent  to  the  machine  shop,  where  they  are 
socketed  for  screw  nipples  and  faced  up  true  at  both  ends.  The 
ease  with  which  graphite  can  be  machined  is  a  distinct  advant- 
age, as  the  electrode  joints  can  be  made  far  more  perfect  than 
with  amorphous  electrodes.  The  commonest  sizes  for  steel 
furnaces  are  4  inches,  5J  inches,  6  inches,  9  inches,  and  12 
inches  in  diameter. 

Chemical  and  Physical  Characteristics. — The  raw  materials 
from  which  graphitised  electrodes  are  made  are  themselves  pure 
forms  of  carbon,  containing,  in  the  case  of  petroleum  coke,  less 
than  '2  per  cent.  ash.  Whatever  the  mineral  content  may  be, 
it  is  almost  entirely  volatilised  during  conversion,  and  the  result- 
ing electrodes  will  always  contain  over  99'5  per  cent,  of  graphitic 
carbon.  Graphitised  electrodes  are  tough  and  resist  combustion 
far  better  than  other  varieties.  The  specific  electrical  resistance 
is  low,  being  only  about  '00030  ohm  per  inch  cube,  but  the 
thermal  conductivity  is  correspondingly  high. 

Electrode  Joints. — Electrodes  which  can  be  fastened  to- 
gether by  means  of  a  simple  male  and  female  screw  joint  are 
now  universally  used  for  the  operation  of  steel  furnaces.  The 
electrical  resistance  between  the  threaded  surfaces  of  the  nipple 
and  socket  is  always  considerable,  so  that  the  safe  cu  rent-carry- 
ing capacity  of  the  joint  will  largely  determine  the  most  suitable 
diameter  for  an  electrode.  Jointing  should  be  done  with  con- 
siderable care,  as  upon  this  will  greatly  depend  the  life  of  an 
electrode.  Since  the  screw  threads  of  most  amorphous  carbon 
electrodes  are  press  formed,  considerable  clearance  has  to  be 
allowed  to  ensure  an  easy  fit ;  this,  however,  does  not  apply  in 
the  case  of  machined  graphite  electrodes.  Different  methods  of 
joining  are  therefore  adopted  to  meet  both  cases. 

Amorphous  Electrodes. — Modern  electrodes  are  manufactured 
with  screw  sockets  at  both  ends,  and  are  joined  together  by 
male  screw  nipples,  which  are  rather  shorter  than  twice  the 
depth  of  a  socket.  To  ensure  good  electrical  contact  between 
the  threaded  surfaces,  it  is  necessary  to  use  a  highly  conduct- 
ing cement,  which  must  be  sufficiently  plastic  to  flow  under 


318  THE    ELECTRO-METALLURGY    OF   STEEL 

uniform  pressure  at  all  points.  The  cementing  material,  which 
has  given  the  best  results  in  practice,  consists  of  very  finely 
pulverised  graphite,  mixed  with  a  pitch-tar  binder  to  give  a 
consistency  of  treacle  at  ordinary  temperatures.  This  paste 
is  well  smeared  over  the  contact  surfaces  before  screwing 
a  nipple  into  the  screw  socket.  If  the  paste  does  not  adhere  to 
the  carbon,  the  surface  should  be  brushed  over  with  the  least 
possible  amount  of  thin  tar  before  application.  Having  securely 
screwed  the  nipple  into  the  upper  socket  of  the  lower  electrode 
in  this  manner,  the  upper  electrode,  with  its  lower  socket  already 
prepared  with  the  graphite  paste,  is  brought  immediately  above 
it  ;  the  butt  ends  and  the  protruding  half  of  the  nipple  are 
then  liberally  smeared  with  paste  before  screwing  down.  The 
upper  electrode  should  be  screwed  down  just  tight  enough  to 
squeeze  out  the  paste  from  between  the  butt  end  faces,  and 
should  not  be  jerked  forward  to  secure  an  exceedingly  tight 
joint.  Experience  proves  that  nipples  show  the  least  tendency 
to  break  when  the  electrode  joint  is  not  absolutely  rigid ;  this 
seems  to  indicate  that  there  is  unequal  expansion  of  the  nipple  and 
electrode,  which  can  only  be  satisfied  without  fracture,  if  the 
joint  is  not  excessively  tight.  Loose  joints,  on  the  other  hand, 
result  in  poor  electrical  contact  and  excessive  local  heating, 
which  may  cause  the  loss  of  an  unconsumed  stub  end  owing  to  ex- 
cessive combustion  and  consequent  thinning  of  the  socket  walls. 
It  is  not  always  an  easy  matter  to  join  ponderous  amorphous 
electrodes,  and  it  requires  considerable  skill  and  judgment  on 
the  part  of  a  crane-driver  to  hold  the  weight  of  the  electrode 
and  lower  it  at  exactly  the  same  rate  as  it  is  fed  downwards  by 
screwing  on  to  the  nipple.  If  the  electrode  is  lowered  too  fast, 
the  full  weight  is  carried  on  the  thread  of  the  nipple  and,  even 
if  no  damage  is  done  to  it,  further  screwing  is  difficult ;  on  the 
other  hand,  if  the  electrode  is  screwed  faster  than  it  is  lowered, 
it  will  bind  on  the  thread  and  may  fracture  the  screw  nipple  if 
the  lower  electrode  is  rigidly  held.  To  overcome  this  difficulty 
and  facilitate  joining,  a  special  screw  plug  (Fig.  128)  has 
been  used,  consisting  of  a  central  plug  screwed  inside  an  outer 
plug  which  fits  into  the  electrode  socket.  The  central  plug  is 
suspended  from  the  crane-hook  and  its  threads  are  cut  to  the 
same  pitch  as  the  socket  thread.  With  this  simple  appliance, 


PEOPERTIES   AND    MANUFACTURE    OF   CARBON   ELECTRODES      319 


the  vertical  feed  downwards  of  the  suspended  electrode  is  exactly 
correct  at  all  times  for  the  degree  of  rotation.  The  plug  should 
be  provided  with  a  safety  locking  device,  which  is  only  released 
when  the  electrode  is  just  above  the  nipple  and  ready  to  be 
screwed  on  to  it. 

It  has  been  proposed  to  improve  the  conductivity  of  the 
cementing  paste  by  the  ad- 
mixture of  metallic  filings, 
but  it  is  doubtful  if  any  ad- 
vantage is  gained,  as  a  suit- 
able paste  made  with  a  high 
grade  graphite,  and  not- 
ordinary  impure  plumbago, 
is  a  better  conductor  than 
the  electrode  material. 

In  many  works  it  is  pre- 
ferred to  join  a  new  elec- 
trode to  another  without 
removal  of  the  short  piece 
from  the  holder,  but  when 
the  headroom  between  the 
electrode-holder  and  the 
crane-hook  will  allow,  it  is 
certainly  quicker  and  more 
advisable  to  have  a  jointed 
electrode  ready  for  replacing 
a  short  length.  In  this 
way  the  join  can  be  made 
more  carefully  than  is  other- 
wise possible  on  a  hot 
furnace  roof. 

Amorphous  electrodes  are  pressed  with  a  -J-inch  taper  to  a  six 
foot  length,  so  that  care  is  therefore  always  taken  before  joining  to 
arrange  for  the  larger  diameter  to  be  uppermost.  This  precau- 
tion is  necessary  to  prevent  an  electrode  slipping  through  a  large 
holder  which  at  times  may  expand  sufficiently  to  relax  its  grip. 

Graphite  Electrodes. — Graphite  electrodes  are  now  uni- 
versally joined  by  using  a  male  nipple  which  screws  into  female 
sockets.  The  screwed  surfaces  are  all  machine  fits,  so  that 


FIG.  128. 


320  THE   ELECTRO-METALLURGY  OF   STEEL 

perfect  joints  can  be  made  without  resorting  to  the  use  of  cements 
or  dry  powder.  The  usual  practice  is  simply  to  screw  the  two 
electrodes  together  until  they  just  touch ;  the  least  strain  to 
secure  a  very  tight  joint  invariably  causes  the  nipples  to  break 
either  at  the  time  of  joining,  or,  what  is  far  worse,  later  during 
service. 

Storage. — Amorphous  electrodes,  owing  to  their  considerable 
porosity,  should  always  be  stored  under  cover,  and  should  be 
raised  clear  of  any  floor  which  is  likely  to  become  damp.  When 
possible,  they  are  best  stored  near  a  drying  stove  or  annealing 
furnace. 

Current -Carry  ing  Capacity  of  Electrodes. — The  safe  current- 
carrying  capacity  of  amorphous  and  graphite  electrodes  cannot 
be  calculated  from  purely  theoretical  figures,  and  must  be  based 
upon  the  results  of  practical  experience.  Sufficient  has  already 
been  said  to  indicate  the  weakness  of  the  screw  joint  and  the 
trouble  that  may  ensue  from  excessive  contact  resistance  and 
from  the  increased  current  density  in  the  nipple  due  to  poor 
contact  between  the  butt  ends  of  the  electrodes. 

The  screw  joint,  therefore,  must  be  regarded  in  the  same  light 
as  the  weakest  link  in  a  chain,  and  will  determine  the  smallest 
and  most  economical  size  of  electrode  which  can  be  used.  If 
the  screw  joint  could  be  ignored,  the  most  economical  diameter 
to  select,  from  a  purely  theoretical  standpoint,  would  be  such 
that  half  the  rate  of  internal  heat  generation  by  electrical  re- 
sistance equals  the  rate  at  which  heat  is  normally  conducted 
from  the  hot  furnace  by  the  electrode  and  radiated  to  the  atmo- 
sphere. For  practical  purposes  the  following  figures  may  be 
regarded  as  typical  of  the  diameters  of  amorphous  and  graphite 
electrodes  commonly  adopted  for  various  current-carrying 
capacities,  due  allowance  having  been  made  for  resistance  of 
joints : — 

Current  in  Amperes.  Amorphous.  Graphite. 

2000-3000  14  inches  5|-   or  6  inches 

3000-4000  14      ,  6  inches 

4000-5000  16 

5000-6000  1 6 

6000-7000  18 

7000-8000  18 


8000-9000  20 

10,000  12,000  20 


9 
9 
10 
12 


PBOPEET1ES  AND   MANUFACTURE   OF   CARBON  ELECTRODES      321 

Comparison    of   Amorphous   and    Graphite  Electrodes. — All 

users  of  electric  steel  furnaces  are  confronted  with  the  question 
of  the  relative  merits  of  amorphous  and  graphite  electrodes. 
A  few  years  ago  amorphous  carbon  held  the  field,  but  recently 
results  with  small  graphite  electrodes  have  shown  such  remark- 
ably low  electrode  consumptions  per  ton  of  steel  that  it  is  not 
surprising  to  find  a  growing  demand  in  their  favour.  Both 
types  have  their  distinct  advantages  and  disadvantages,  and  it 
will  be  as  well  before  considering  these  in  detail  to  compare 
briefly  their  physical  properties  as  given  by  the  Acheson 
Graphite  Co.  in  one  of  their  publications:— 

Amorphous  Carbon         Graphite 

Electrodes.  Electrodes. 

Specific  resistance  ohms  per  in.  cube         .         .       -00124  -000320 

Specific  resistance  ohms  per  cm.  cube        .         .       -00315  -000813 

Comparative  sect,  area  for  same  voltage  drop   .3-8  1 

Weight :  Ib.  per  cube  in -0564  -0574 

Weight:  grms.  per  cu.  em.         ....     1-56  1-59 

Density 2-00  2-21 

Tensile  strength,  Ib.  per  sq.  in 1000  to  5000  800  to  1000 

Temp,  of  oxidation  in  air 500°  C.  640°  C. 

Apart  from  the  relative  merits  indicated  in  the  above  table 
from  a  purely  physical  standpoint,  there  are  others,  which, 
although  not  apparent  from  actual  electrode  consumption,  may 
indirectly  influence  the  total  manufacturing  cost  per  ton  of  steel. 
For  this  reason  an  electrode  consumption  figure,  which  may  be 
actually  very  small,  should  not  always  be  regarded  as  an  in- 
dispensable factor  towards  economy  of  production.  It  is  obvi- 
ously worthless  to  draw  a  comparison  between  the  electrode 
consumption  of  two  furnaces  of  equal  load  capacity,  one  having 
perhaps  three  amorphous  carbon  electrodes  and  the  other  only 
one  of  graphite.  In  this  case  the  undoubted  economy  of 
electrode  consumption  in  favour  of  the  graphite  electrode  is  due 
solely  to  factors  which  are  entirely  independent  of  the  relative 
behaviour  under  exactly  similar  conditions.  Therefore,  to  draw 
a  true  comparison  between  the  two  varieties,  it  is  necessary  to 
use  them  in  furnaces  of  similar  design  operating  under  the  same 
working  conditions. 

Effect  of  Combustion. — Consumption  of  electrodes  by  natural 
combustion  inside  and  outside  the  furnace  is  naturally  greater 

21 


322  THE    ELECTEO-METALLURGY    OF    STEEL 

in  the  case  of  amorphous  carbon.  For  the  same  current-carry- 
ing capacity,  the  diameter  is  larger,  and  to  this  must  be  added 
the  far  greater  tendency  of  the  amorphous  carbon  grains  to 
burn.  Hot  joints  are  far  more  frequent  and  pronounced  with 
amorphous  electrodes,  and  sometimes  account  for  considerable 
wastage  both  by  combustion  and  loss  of  stub  ends.  Graphite 
electrodes  are  not  tapered,  and,  owing  also  to  their  compara- 
tively small  size,  are  more  readily  adaptable  to  the  several  gas 
sealing  devices  now  used,  and  also  to  the  cruder  method  of 
blocking  the  annular  opening  by  the  liberal  use  of  ganister. 
For  this  reason,  the  combustion  of  graphite  electrodes  above 
the  roof  may  be  more  readily  prevented  than  in  the  case  of 
amorphous  carbon. 

Loss  by  Fracture. — Amorphous  carbon  has  a  higher  tensile 
strength  than  graphite,  and  for  equal  current-carrying  capacity 
requires  a  sectional  area  at  least  four  times  as  great.  There- 
fore, an  amorphous  electrode  of  equivalent  conductivity  would 
be  at  least  four  times  stronger  than  graphite,  and  offer  far  less 
risk  of  fracture  by  accidental  blows  received  during  manipulation 
of  the  furnace.  This  is,  of  course,  assuming  the  amorphous 
electrode  to  be  of  good  quality  and  not  subject  to  the  defects  due 
to  improper  grain  size  and  volatile  matter. 

The  amorphous  electrode  screw-joint  has  a  liberal  margin 
of  strength  and  will  usually  withstand  any  light  blows  given  to 
the  lower  end  of  the  electrode.  On  the  other  hand,  the  weak- 
ness of  the  screw-joint  of  graphite  electrodes  is  often  a  serious 
drawback ;  this  results  from  the  small  diameter  of  the  nipple, 
which,  although  strong  enough  to  support  the  weight  of  the 
stub,  is  easily  fractured  by  side  blows.  To  prevent  a  high  con- 
sumption with  graphite  electrodes  it  is  essential,  therefore,  to 
charge  the  furnace  with  considerable  care,  more  especially  when 
heavy,  irregular  shaped  scrap  is  being  used.  The  danger  of  a 
nipple  breaking  is  not  so  apparent  in  the  larger  sizes  of  9  inches 
diameter  or  more  as  in  those  of  small  diameter. 

The  fracture  of  a  joint  always  causes  a  delay  occasioned  by 
removal  of  the  broken  piece  from  the  furnace  and  replacement 
of  a  new  length;  sometimes  also  a  bath  of  steel  may  be 
carburised  by  contact  with  the  electrode,  in  which  case  the 
"  heat  "  will  be  further  delayed  by  boiling  down.  Such  dim- 


PROPERTIES   AND   MANUFACTURE   OF   CARBON   ELECTRODES      323 

culties  are  common  to  both  types,  but  there  is  a  slight  advant- 
age in  favour  of  graphite  electrodes,  which,  by  reason  of  their 
much  smaller  diameter,  can  be  removed  more  expeditiously  and 
with  less  risk  of  disturbing  the  metallurgical  conditions.  Apart 
from  the  difficulties  of  removal,  the  fracture  of  a  nipple  joining 
two  amorphous  electrodes  results  in  complete  loss  of  the  stub- 
end.  This  is  not  necessarily  the  case  with  graphite  electrodes, 
since,  with  a  little  care  and  patience,  the  halves  of  the  broken 
nipple  can  often  be  removed  from  their  screw  sockets  and  the 
same  electrodes  joined  together  again  by  a  fresh  nipple. 

Effect  of  Weight  and  Size. — Graphite  electrodes,  by  virtue  of 
their  lighter  weight  and  smaller  diameter,  cause  less  trouble  with 
holders  than  the  heavy  amorphous  electrodes.  A  reduced 
diameter  is  also  a  point  of  great  advantage  in  roof  construc- 
tion and  its  ultimate  life.  On  the  other  hand,  amorphous 
electrodes  of  large  diameter  shield  the  roof  to  a  far  greater 
extent  from  the  heat  directly  radiated  from  the  arc,  which  is  a 
point  of  considerable  importance,  and  has  a  great  bearing  upon 
the  life  of  a  furnace  lining  and  the  cost  of  repairs.  Excessively 
high  roof  temperatures  generally  lead  to  metallurgical  difficulties, 
more  especially  in  basic  lined  furnaces,  and  this  introduces  a 
further  element  affecting  the  one  figure  which  can  alone  be 
indicative  of  comparative  economy,  namely,  the  over-all  manu- 
facturing cost  of  producing  one  ton  of  steel  in  the  ladle. 

Eelative  Cost. — The  question  of  the  relative  economy  of 
graphite  and  amorphous  electrodes  is  very  involved,  and  can 
only  be  satisfactorily  settled  by  a  comparison  of  the  manufactur- 
ing costs  obtained  from  two  furnaces  of  the  same  type,  operating 
under  similar  conditions,  but  fitted  with  the  two  varieties  of 
electrodes.  Figures  of  electrode  consumption  per  ton  of  steel 
convey  little  beyond  a  comparison  of  the  standard  of  excellence 
attained  in  different  furnaces  of  similar  design,  operating  under 
similar  conditions. 

If  the  relative  cost  of  amorphous  and  graphite  electrodes  is 
based  solely  on  the  consumption  per  ton  of  steel,  it  will  be 
generally  found  that  the  cost  is  about  the  same  in  either  case, 
provided  economisers  are  used  with  the  graphite  electrodes. 
The  market  prices  of  the  two  products  are  still,  however,  very 
variable,  and  until  conditions  become  more  settled,  it  is  not 


324  THE    ELECTEO-METALLUEGY   OF    STEEL 

possible  to  say  which  will  eventually  be  the  cheaper  form  to 
use. 

Before  the  war  the  principal  source  of  amorphous  electrodes 
was  Germany,  though  works  were  in  operation  in  the  United 
States,  Sweden,  France,  and  Italy.  Graphite  electrodes  were 
only  produced  in  the  United  States.  During  the  war  four 
amorphous  and  one  graphite  electrode  factory  were  erected  in 
England,  the  former  having  a  total  capacity  of  about  16,000 
tons  and  the  latter  of  about  1000  tons  per  annum.  A  large 
proportion  of  the  amorphous  electrodes  are  used  for  the  manu- 
facture of  ferro-alloys  and  carbide,  and  of  the  graphite  electrodes 
for  the  electro-chemical  industry.  Great  Britain,  accordingly, 
now  produces  not  only  the  greater  part  of  the  electrodes  con- 
sumed in  this  country,  but  exports  a  large  tonnage  to  Scandinavia 
and  other  countries. 

Electrode  Economisers. — It  must  be  acknowledged  that  the 
escape  of  hot  and  sometimes  combustible  gas  through  the 
annular  openings  between  electrodes  and  the  furnace  lining  is 
an  objectionable  feature  of  furnace  operation.  Not  only  is  the 
consumption  of  electrodes  augmented,  but  the  life  of  the  lining 
is  also  influenced.  There  are,  therefore,  great  advantages  to  be 
gained  by  effectively  sealing  the  electrode  openings,  some  of 
which  have  been  already  mentioned  in  connection  with  electric 
furnace  design. 

The  problem  is  far  more  difficult  than  would  appear  at  first 
sight  and  is  materially  affected  according  to  whether  amorphous 
or  graphite  electrodes  are  employed.  The  former  type  when 
mould  pressed  is  tapered,  so  that,  apart  from  the  effects  of  sur- 
face combustion,  the  annular  opening  will  be  constantly  varying 
in  width.  Local  heating  in  the  neighbourhood  of  joints  is  a 
common  occurrence,  and  often  results  in  combustion  to  a  markel 
degree,  even  while  the  joint  is  still  outside  the  furnace  lining  or 
cooling  jacket.  Again,  the  screw  sockets  into  which  the  male 
nipples  are  screwed  are  moulded  in  the  electrodes,  and,  as  ample 
clearance  has  also  to  be  allowed  between  the  threads  of  the 
socket  and  nipple,  it  is  not  unusual  to  find  an  overlap  of  a  J  inch 
at  the  joint  between  two  electrodes. 

On  the  other  hand,  graphite  electrodes,  which  have  been 
originally  formed  by  extrusion,  are  cylindrical  and  comparatively 


PROPERTIES  AND   MANUFACTURE   OF   CARBON   ELECTRODES     325 

free  from  surface  deformations.  The  sockets  and  nipples  are 
machine  threaded  to  make  a  perfect  fit,  and,  if  the  electrodes 
are  carefully  mounted  when  boring  there  is  no  reason  for  any 
overlap  in  joining  two  electrodes  together  ;  in  practice,  however, 
there  is  sometimes  an  overlap  not  exceeding  one-eighth  of  an 
inch.  Although  local  heating  at  the  joints  is  by  no  means  in- 
frequent, the  diameter  will  not  be  visibly  reduced  by  combus- 
tion, at  all  events  until  the  joint  is  situated  well  within  the 
furnace. 

There  are  two  possible  methods  of  sealing  the  annular  open- 
ing between  an  electrode  and  the  furnace  body  : — 

I.  By  the  use  of  a  sealing  sleeve,  gland,  or  collar. 

II.  By   totally   enclosing    the   electrode   within   a    flexible 
jacket  connecting  the  furnace  body  to  the  electrode  holder. 

Whatever  device  may  be  used,  it  need  not  entirely  prevent 
the  escape  of  gas,  provided  that  its  volume  and  temperature  are 
both  reduced  to  limits  which  will  render  it  incapable  of  causing 
excessive  combustion  of  the  electrode,  or  of  harmfully  affecting 
the  life  of  the  lining. 

To  appreciate  the  exact  nature  and  extent  of  the  duties 
which  a  satisfactory  economiser  is  called  upon  to  perform,  it  is 
first  necessary  to  understand  clearly  the  manner  in  which  com- 
bustion of  the  electrodes  is  promoted  by  the  escape  of  gases 
through  unrestricted  annular  openings,  and  from  other  causes. 
Combustion  may  proceed  both  inside  and  outside  the  furnace 
body.  In  the  former  case,  the  annular  passages  act  like  chimney 
flues  and  so  induce  currents  of  air,  which  enters  the  furnace 
through  loosely  fitting  doors  and,  becoming  heated,  rises  up- 
wards and  finally  escapes  from  around  the  electrodes.  Combus- 
tion, therefore,  proceeds  over  the  entire  surface  exposed  to  the 
action  of  the  air  currents,  and  is  naturally  intensified  at  the 
points  of  escape  where  the  velocity  of  the  gases  is  at  a  maximum. 
It  is  evident  then  that  simple  combustion  caused  by  the  oxidis- 
ing action  of  such  induced  currents  of  air  may  be  minimised 
by  carefully  sealing  all  door  openings — which  is  difficult  in 
practice — or  by  preventing  the  escape  of  gas  through  the  elec- 
trode openings. 

The  combustion  of  an  electrode  over  an  extended  region 
outside  the  furnace  body  may  be  caused  by  the  escape  of 


326  THE   ELECTRO-METALLURGY  OF   STEEL 

combustible  furnace  gases,  which,  burning  at  their  point  of  issue, 
heat  the  electrode  above  its  combustion  temperature.  Apart 
also  from  the  heating  effect  of  such  gases,  an  electrode,  by  virtue 
of  its  thermal  conductivity,  or  by  reason  of  its  slow  or  rapid 
withdrawal  from  a  hot  furnace,  or  of  electrical  resistance  heating, 
may  be  at  a  temperature  above  that  of  combustion  for  a  distance 
of  several  inches  beyond  the  furnace  body  or  cooling  jacket ;  for 
this  reason  it  becomes  necessary  to  protect  such  a  highly  heated 
zone  from  the  action  of  the  surrounding  air. 

The  essential  features  of  a  satisfactory  economiser  can  now 
be  enumerated  : — 

I.  The  annular  space  between  the  furnace  body  and  the  elec- 
trode should  either  be  closed,  or  so  restricted  that  the  volume 
of  gas  capable  of  escape  at  any  time  is  too  small  to  induce  harm- 
ful currents  of  air  through  the  furnace. 

II.  If  the  annular  passage  is  not  entirely  closed,  the  gas 
still  capable  of   escape   should   be   cooled    during   its  passage 
through  the  economiser  to  a  temperature  below  that  of  combus- 
tion before  finally  mixing  with  the  surrounding  atmosphere. 

III.  Highly   heated   portions   of    an    electrode   outside   the 
furnace  lining  should  be  protected  from  the  surrounding  atmos- 
phere. 

IV.  The  apparatus  should  be  inexpensive  and   capable  of 
rapid  and  easy  removal. 

V.  No  added  difficulties  should  be  encountered  in  the  event 
of  an  electrode  joint  breaking  at  a  time  when  the  lower  electrode 
does  not  fall  clear  of  the  roof  and  so  cannot  be  removed  except 
by  withdrawal  outwards  through  the  roof  openings. 

VI.  The  apparatus  should  be  self-adjusting  and  its  opera- 
tion entirely  independent  of  the  relative  positions  of  the  electrode 
axis  and  the  roof,  which  are  constantly  changing.     In  the  case 
of  a  roof  that  rises  badly,  the  surface  of  the  cooler,  originally 
normal  to  the  axis  of  the  electrode,  will  often  be  inclined  at  an 
angle  of  10°  or  15°. 

VII.  Weight  should   be   reduced  to   a  minimum   to  avoid 
added  pressure  on  the  roof  as  far  as  possible. 

VIII.  The   total    height   of    the  apparatus   should   on    no 
•account  seriously  reduce  the  effective  travel  of  an  electrode  as 

originally  provided  for. 


PROPERTIES  AND   MANUFACTURE   OF   CARBON   ELECTRODES      327 

IX.  The  apparatus   should  be  constructed  so  as  to  with- 
stand moderately  rough  usage,  and  its  life  should  be  compatible 
with  its  initial  cost. 

X.  Simplicity  should  be  aimed  at  and  complexity  of  parts 
avoided. 

XI.  Overlapping  joints,  hot  joints,  surface  deformations  and 
"  necking  "  at  joints  should  be  quite  incapable  of  causing  damage 
to  the  economiser,  although  in  the -latter  case  its  effective  opera- 
tion may  be  somewhat  impared. 

Several  types  of  economisers  have  been  introduced,  but  it 
must  be  admitted  that  perfection  has  not  been  reached,  and 
probably  never  will  be  unless  certain  desirable  features  of  fur- 
nace construction  are  entirely  subordinated 
to  the  one  paramount  object  of  sealing  the 
electrode  openings. 

The  desirability  of  closing  the  annular 
openings  was  realised  by  the  earliest  furnace 
designers,  the  use  of  the  simple  water-cooled 
ring  being  introduced  by  Heroult  as  early  as 
1903.  These  cooling  rings,  which  have 
been  so  universally  adopted,  are  constructed  Flo  129 

to  allow  a  minimum  clearance  of  about  three- 
quarters  of  an  inch  around   the  electrode.     They  are   always 
very  shallow,  being  only  about  3  inches  high,  and  so  do  not 
offer  any  material  resistance  to  the  passage  of  escaping  gases. 

Stobie  in  1916  made  further  attempts  to  restrict  the  passage 
of  gases,  and  also  to  prevent  the  combustion  of  an  electrode  in 
that  zone  where  the  temperature  is  sufficiently  high  to  promote 
it.  The  economiser  which  is  shown  in  Fig.  129  consists  of  a 
tubular  metal  jacket  resting  upon  the  roof  and  enclosing  the 
electrode.  The  height  is  so  determined  that  the  portion  of  the 
electrode  projecting  above  the  jacket  is  never  likely  to  become 
heated  above  the  temperature  of  combustion.  The  upper 
extremity  of  the  annular  opening  is  closed  by  suitable  packing, 
which  rests  loosely  upon  the  jacket ;  any  gas  that  may  pass 
through  the  loosely  fitting  gland  will  be  below  combustion 
temperature  and  so  rendered  harmless.  This  simple  apparatus 
is  effective,  provided  the  diameter  of  the  electrode  remains  fairly 
uniform.  A  reduced  diameter  at  the  gland  will  allow  more  gas 


328 


THE   ELECTEO-METALLUEGY   OF    STEEL 


to  escape  ;  this  can  only  increase  the  rate  of  combustion  of  the 
electrode  inside  the  furnace  by  causing  a  greater  indraught  of 
air,  unless  the  gas  itself  burns  at  its  point  of  exit,  when  external 
combustion  as  well  would  be  promoted.  The  clearance  between 
the  electrode  and  the  jacket  is  considerable,  which  allows  for 
moderate  displacement  of  the  latter  without  interfering  with 
its  effective  operation.  The  height  of  the  economiser  is  such 
that  it  cannot  be  applied  to  existing  furnaces  having  a  limited 
range  of  electrode  travel.  The  Stobie  furnace  is  designed  with 
a  rather  more  elevated  raising  gear,  so  that  ample  electrode 
travel  is  provided  for.  Excellent  results  have  been  obtained  and, 
according  to  Stobie,  the  electrode  consumption  per  ton  of  steel 
has  been  reduced  to  6  Ib.  with  a  Stobia  furnace  having  four 
graphite  electrodes.  An  economiser  of  this  design  is  far  more 
effective  with  graphite  than  with  amorphous  electrodes,  and  will 


FIG.  130. 


FIG.  131. 


not  satisfactorily  meet  the  conditions  raised  by  high  local  heat- 
ing at  the  joints  between  those  of  the  latter  variety.  Stobie  has 
also  designed  another  type  of  economiser,  which  takes  the  form  of 
a  telescopic  jacket  entirely  enclosing  the  electrode  between  the 
furnace  roof  and  the  holder.  All  those  objections  inherent  to 
the  gland  type,  such  as  overlapping  joints,  "  necked  "  joints  and 
natural  taper,  at  once  disappear,  and,  furthermore,  this  design  is 
more  adaptable  to  existing  furnaces  having  a  limited  electrode 
travel,  the  minimum  height  of  the  telescopic  jacket  being  less 
than  that  of  the  tubular  type. 

Several  attempts  have  been  made  to  produce  a  simple  shallow 
type  of  economiser,  acting  on  the  principle  of  a  flexible,  self- 
adjusting  gland,  which  could  be  used  on  any  furnace  whose 
design  does  not  embody  a  specially  elevated  electrode  raising 
gear.  The  economiser  shown  in  Fig.  130  was  introduced  for 


PROPERTIES  AND   MANUFACTURE   OF   CARBON   ELECTRODES     329 

use  with  the  Electro-metals  furnace.  The  apparatus  consists 
of  four  separate  glands,  each  gland  being  composed  of  three  iron 
segments,  which  are  free  to  move  independently  of  one  another, 
and  always  tend  to  butt  against  the  electrode.  The  construc- 
tion of  the  glands  imparts  flexibility  of  movement,  and  enables 
the  gas  sealing  device  to  accommodate  itself  to  small  changes  in 
the  diameter  of  an  electrode,  and  to  axial  displacement  due  to 
roof  distortion. 

Another  type  of  economiser,  based  upon  rather  different 
principles  of  operation  to  the  foregoing,  is  shown  in  Fig  131. 
This  apparatus  is  intended  to  check  the  flow  of  gas  to  a  volume 
incapable  of  inducing  air  currents  of  harmful  proportions.  The 
sleeve,  which  is  made  from  a  refractory  material  of  high  thermal 
conductivity,  is  also  designed  to  cool  down  the  small  quantity 
of  gas  to  a  temperature  below  that  of  its  ignition.  Further- 
more, the  portion  of  the  electrode  outside  the  furnace  body, 
which  is  often  inevitably  above  the  temperature  of  combustion, 
is  protected  from  the  surrounding  atmosphere  by  the  close 
fitting  sleeve,  which  extracts  the  heat  rapidly  by  virtue  of  its 
high  thermal  conductivity  and  large  surface  of  radiation.  The 
use  of  a  close  fitting  sleeve,  which  will  retain  an  unbroken  joint 
between  the  furnace  roof  and  itself,  irrespective  of  roof  distortion, 
is  made  possible  by  mounting  the  sleeve  upon  a  ring,  with  which 
it  makes  a  ball  and  socket  union.  The  seating  ring  rests  upon 
the  usual  cooling  box,  or  a  plain  ring  of  the  same  refractory 
material,  and  in  either  case  is  free  to  slide  laterally  in  all 
directions.  The  sleeve  will  retain  an  unbroken  joint  at  its 
seating,  provided  the  inclination  of  the  cooling  box  to  its  original 
position,  normal  to  the  axis  of  the  electrode,  does  not  exceed 
15° ;  this  angle  allows  for  the  worst  possible  degree  of  roof  dis- 
tortion. It  is  obvious  that  ttu's  type  of  economiser  is  more 
effectual  when  using  graphite  electrodes,  which  do  not  present 
the  same  difficulties  as  the  amorphous  variety,  as  previously 
mentioned.  The  height  of  the  sleeve  is  such,  however,  that, 
even  when  considerable  necking  at  a  joint  has  taken  place,  there 
will  always  be  a  full  diameter  section  of  the  electrode  enclosed 
within  it,  so  that  the  flow  of  gas  is  nevertheless  restricted, 
although  to  a  lesser  extent. 


APPENDIX. 

METHODS  OF  BATH  SAMPLING  AND  RAPID  ANALYSIS. 

THE  great  importance  attached  to  the  rapid  working  of  electric 
furnaces,  uninterrupted  by  delays  due  to  mechanical  or  metal- 
lurgical causes,  has  been  emphasised  in  those  chapters  relating 
to  their  technical  and  economic  operation.  It  has  also  been 
shown  that  the  process  of  steel-making  is  generally  conducted 
in  electric  furnaces  under  careful  chemical  control,  so  that 
chemical  analysis  becomes  of  great  importance. 

•It  is  seldom  attempted  to  produce  steel  to  a  specified  analysis 
by  a  single  addition  of  ferro-alloys  and  carburising  material  to 
a  bath  of  undetermined  chemical  composition.  The  general 
practice  is  to  carburise  in  two  stages  and  to  determine  the 
carbon,  and  sometimes  also  the  manganese  content  of  the  bath, 
after  the  preliminary  carbon  addition  and  at  a  time  when 
further  variation  of  composition  is  unlikely.  This  method  pre- 
sents an  accurate  means  of  calculating  the  final  additions 
necessary  for  bringing  the  analysis  within  the  specification 
limits. 

In  the  manufacture  of  alloy-steels,  the  raw  materials  often 
consist  of  scrap  steel  containing  one  or  more  of  the  alloying  metals 
required  in  the  final  steel,  and  it  is  then  essential  to  know  the 
percentage  of  these  constituents  in  the  bath  at  the  same  time 
that  a  bath  sample  is  taken  for  the  analysis  of  carbon.  This 
enables  correct  final  additions  of  alloying  metals  to  be  made 
simultaneously  with  the  final  adjustment  of  the  carbon  content. 
It  is  unfortunate  that  practically  every  metallic  constituent  of 
any  alloy  steel  is  subject  to  variation  until  the  chemical  con- 
dition of  both  the  bath  of  steel  and  slag  are  such  that  no  further 
reaction  involving  oxidation  or  reduction  of  metals  is  possible. 
When  such  a  stage  has  been  reached,  either  in  the  basic  or  acid 
process,  the  steel  would  be  normally  nearly  ready  for  tapping, 

(330) 


APPENDIX  331 

provided  the  quantity  of  subsequent  additions  were  known. 
For  this  reason  it  is  obvious  that  the  bath  sampling,  preparation 
of  the  sample,  and  the  analysis  of  drillings  should  all  be  done 
with  the  least  possible  delay  without  sacrificing  the  care  neces- 
sary both  in  sampling  and  analysis.  The  use  of  rapid,  accurate 
methods  of  analysis,  based  as  far  as  possible  on  volumetric 
principles,  is  therefore  invaluable.  In  this  Appendix,  following 
a  brief  description  of  bath  sampling  and  sample  preparation, 
a  selection  of  rapid  and  sufficiently  reliable  methods  of  analysis 
are  given.  All  these  methods  have  been  widely  used  for  con- 
trolling the  analyses  of  electric  steels,  and  in  some  cases  represent 
the  standard  practice  in  most  steel  works  laboratories. 

Preparation  of  Sample. — A  bath  of  steel  is  sampled  by  with- 
drawing a  small  quantity  in  a  special  sampling  spoon  and  pour- 
ing into  an  iron  mould. 

The  spoon  (see  Fig.  81)  should  be  warmed  and  covered  with  a 
coating  of  slag  before  immersion  into  the  steel,  this  being  done 
to  prevent  steel  freezing  on  to  it.  It  is  convenient  to  add  a 
very  small  piece  of  aluminium  to  the  mould  or  to  the  steel  in 
the  spoon  before  pouring,  so  that  the  sample  may  be  sound  and 
more  suitable  for  drilling.  When  sampling  a  bath  of  steel  after 
an  addition  of  a  carburiser,  ferro-alloy,  or  an  alloying  metal,  it 
is  most  important  to  ensure  that  the  spoon  sample  is  representa- 
tive of  a  homogeneous  bath,  and  for  this  reason  a  preliminary 
rabbling  is  given  with  a  heavy  slag-covered  skimming  tool  or 
bar.  The  sample  ingot  may  be  either  cylindrical  or  rectangular 
in  shape,  but  should  be  capable  of  being  firmly  held  under  the 
drill.  If  the  underside  is  convex,  the  sample  should  be  flattened 
under  a  hammer  while  still  at  a  forging  temperature,  and  centre- 
punched  later  to  facilitate  drilling.  Samples  of  steels  subject 
to  water  or  air  hardening  are  either  allowed  to  cool  down  to 
below  redness  in  air  before  quenching  in  water,  or  may  be  more 
rapidly  cooled  by  slowly  quenching  the  side  remote  from  the 
face  to  be  drilled.  The  rapid  cooling  of  water-hardening  steels 
in  this  latter  way  must  be  done  with  caution,  otherwise  con- 
siderable delay  may  be  caused  by  inability  to  drill  the  sample. 

A  heavy  power  driven  drilling  machine  should  always  be 
available  for  use,  and  should  be  kept  clean  and  free  from  oil  for 
the  special  purpose  of  preparing  samples.  Surface  drillings  are 


332  THE  ELECTRO-METALLURGY  OF  STEEL 

always  rejected  to  prevent  contamination  of  the  clean  drillings 
with  scale  or  dirt. 

Methods  of  Analysis. 

Phosphorus  Determination. — The  determination  of  phos- 
phorus in  a  bath  of  steel  is  only  necessary  when  the  extent  of 
phosphorus  removal  by  an  oxidising  basic  slag  is  uncertain  ; 
such  cases  may  arise  when  melting  cold  charges  of  miscellaneous 
wrought-iron,  cast-iron,  and  steel  scrap  containing  high  and 
irregular  percentages  of  phosphorus.  The  sample  is  taken  and 
analysed  before  removal  of  the  dephosphorising  slag,  and  an 
allowance  should  be  made  for  a  slight  subsequent  rise  as  shown 
on  analysis  of  the  pit  sample.  The  observed  difference  in  the 
phosphorus  contents  of  samples  taken  before  and  after  deoxida- 
tion  of  the  steel  bath  cannot  be  accounted  for  by  the  possible 
reduction  of  any  phosphorus  bearing  slag  remaining  in  the 
furnace  after  skimming.  If  the  phosphorus  found  in  an 
oxidised  bath  sample  is  about  '010  per  cent,  the  phosphorus 
content  of  the  finished  steel  will  generally  be  nearer  '02  per 
cent.,  even  after  the  most  perfect  skimming  possible.  The 
following  method  of  phosphorus  determination  is  rapid  and 
very  widely  used. 

Principle  of  Method. — The  method  consists  of  a  rapid  pre- 
cipitation of  the  phosphorus  as  ammonium  phospho-molybdate, 
which,  instead  of  being  collected,  dried,  and  weighed,  is  washed 
clean  and  dissolved  in  a  solution  of  caustic  soda.  The  exact 
weight  of  phosphorus  in  this  precipitate  is  determined  volu- 
metrically,  from  which  the  percentage  in  the  sample  of  steel 
can  be  accurately  calculated.  The  reaction  takes  place  accord- 
ing to  the  following  equation  :— 

2[(NH4)312Mo03P04]  +  46NaOH  +  H20  =  2[(NH4)2HPOJ  + 
(NH4),Mo04  +  23Na2Mo04  +  23H20. 

Standard  Solutions.  —  (a)  Ammonium  Nitro-Molybdate 
Solution. — Mix  150  grins,  pure  Mo03  with  105  c.c.  strong 
ammonia  and  315  c.c.  water.  Stir  and  filter  off  any  residue. 
Pour  the  filtrate  with  constant  stirring  into  1875  c.c.  of  nitric 
acid  (1*2  sp.  gr.). 

(6)  Caustic  Soda  Solution — To  100  grms.  of  pure  NaOH 


APPENDIX  333 

add  sufficient  water  to  dissolve  all  but  a  few  grms.  ;  the 
undissolved  residue,  which  will  contain  any  sodium  carbon- 
ate, is  allowed  to  settle  and  the  decanted  liquid  diluted  to 
2000  c.c. 

The  phosphorus  equivalent  to  1  c.c.  of  the  caustic  soda  solution 
after  making  it  equivalent  to  the  nitric  acid  solution  is  deter- 
mined by  applying  the  method  of  analysis  to  a  standard  sample 
of  known  phosphorus  content  in  exactly  the  same  manner  as 
described  later  for  the  ordinary  samples. 

(c)  Nitric  Acid   Solution. — Make  up  a  solution  containing 
about  20  c.c.  strong  nitric  acid  (1/42  sp.  gr.)  diluted  to  2000  c.c. 

This  or  the  caustic  soda  solution  should  then  be  diluted  so 
that  1  c.c.  of  acid  exactly  neutralises  1  c.c.  of  soda,  using  an 
alcoholic  solution  of  phenol-phthalein  as  an  indicator ;  this  being 
done  as  follows  : — 

Dilute  10  c.c.  of  the  soda  solution  in  a  conical  flask  to  250 
c.c.  and  add  a  few  drops  of  the  phenol-phthalein  solution ;  run 
in  the  acid  slowly  from  a  burette  until  the  pink  colour  just  dis- 
appears. From  the  number  of  c.c.  of  acid  required  it  is  easy  to 
calculate  the  quantity  of  water  to  add  to  either  the  acid  or  soda 
solution  to  make  them  exactly  equivalent. 

This  preliminary  equalising  of  the  two  solutions  is  not  abso- 
lutely necessary,  but  simplifies  the  calculation  for  each  deter- 
mination of  phosphorus. 

(d)  Phenol-Phthalein  Indicator. — Dissolve  1  grm.  phenol- 
phthalein  in  50  c.c.  of  alcohol. 

Method  of  Operation. — Dissolve  2  grms.  of  drillings  in  75 
c.c.  nitric  acid  (11  sp.  gr.)  in  a  conical  flask  and  boil  off  all  red 
fumes.  Add  a  3  per  cent,  solution  of  potassium  permanganate 
to  produce  a  permanent  pink  coloration  or  a  slight  brown  pre- 
cipitate on  boiling  for  a  few  minutes.  Kemove  the  flask,  add 
just  sufficient  H2S04  to  clear  the  precipitate,  and  then  cool.  Add 
13  c.c.  strong  ammonia  and  50  c.c.  of  molybdate  solution,  shake 
vigorously  and  allow  the  precipitate  to  settle  for  a  few  minutes 
in  a  warm  place. 

Filter  and  wash  the  precipitate  with  2  per  cent,  nitric  acid 
solution,  then  with  a  2  per  cent.  KNO3  solution  until  free  of 
acid.  The  flask  should  also  be  washed  clean  with  KN03,  the 
washings  being  passed  through  the  same  filter. 


334  THE   ELECTRO-METALLURGY    OF   STEEL 

Transfer  the  precipitate  on  the  filter  paper  to  the  same  flask 
and  add  a  sufficient  and  known  quantity  of  soda  solution  to 
dissolve  it ;  dilute  to  about  50  c.c. ,  add  a  few  drops  of  phenol- 
phthalein  indicator  and  titrate  with  the  nitric  acid  solution 
until  the  pink  colour  is  discharged. 

By  deducting  the  number  of  c.c.  of  acid,  or  its  equivalent  of 
soda  solution,  from  the  amount  of  soda  solution  added  to  dis- 
solve the  precipitate,  the  number  of  c.c.  of  the  soda  solution 
actually  consumed  by  the  reaction  with  the  phosphorus  pre- 
cipitate is  known.  Having  determined  the  phosphorus  equiva- 
lent of  the  soda  solution  by  using  a  steel  sample  of  known 
phosphorus  content  the  per  cent,  of  phosphorus  in  the  steel  can 
be  directly  calculated. 

Manganese  Determination. — The  manganese  remaining  in 
a  bath  of  steel  after  the  oxidising  slag  reactions  have  ceased 
may  vary  considerably  according  to  the  character  of  the  scrap 
used  in  the  furnace  charge,  and  may  amount  at  times  to  a 
considerable  part  of  the  specification  percentage.  In  such 
cases  it  is  necessary  to  determine  the  manganese  remaining  in 
the  bath  at  a  time  when  there  is  no  likelihood  of  either  increase 
by  the  reduction  of  manganese  oxide  in  the  slag  or  removal 
under  oxidising  slag  conditions. 

Principle  of  Method. — The  steel  is  dissolved  and  the  solution 
strongly  oxidised  to  destroy  all  carbonaceous  residue. 

The  solution  is  then  reduced  to  destroy  all  higher  oxides  of 
manganese  and  chromium,  if  any.  The  cooled  solution  is  again 
strongly  oxidised,  filtered,  and  reduced  by  a  measured  excess  of 
a  standard  solution  of  ferrous  ammonium  sulphate.  The  unoxi- 
dised  excess  of  the  latter  is  titrated  with  a  standard  perman- 
ganate solution,  whose  equivalent  value  in  terms  of  the  ferrous 
ammonium  sulphate  solution  has  been  determined  ;  the  amount 
of  permanganate  solution  equivalent  to  the  ferrous  ammonium 
sulphate  oxidised  by  the  steel  solution  can  then  be  calculated. 
The  value  of  1  c.c.  permanganate  in  terms  of  manganese  is 
separately  determined  by  applying  the  method  to  a  sample  of 
steel  of  known  manganese  content.  Having  obtained  this  figure, 
the  amount  of  permanganate  equivalent  to  the  ferrous  ammonium 
sulphate  originally  oxidised  by  the  permanganate  formed  from 
the  steel,  multiplied  by  the  determined  manganese  equivalent, 


APPENDIX  335 

gives  the  weight  of  manganese  in  the  weighed  sample,  from 
which  the  percentage  in  the  steel  is  calculated. 

Chromium  below  1*5  per  cent,  does  not  affect  the  accuracy 
of  the  method,  provided  that  the  second  oxidation  of  the 
reduced  solution  is  done  when  cold.  Chromium  then  remains 
in  the  reduced  state,  and  so  has  no  influence  on  the  ferrous 
solution  added  later. 

Standard  Solutions. — (a)  Potassium  Permanganate  Solu- 
tion.— Dissolve  316  grms.  KMnO4  and  dilute  to  one  litre,  mak- 
ing a  decinormal  solution. 

(li)  Ferrous  Ammonium  Sulphate  Solution. — Dissolve  39'6 
grms.  ferrous  ammonium  sulphate  in  3  per  cent,  sulphuric  acid 
and  make  up  to  2000  c.c.,  using  the  same  acid.  This  solution 
is  also  decinormal  and  1  c.c.  should  exactly  satisfy  1  c.c.  of  the 
permanganate. 

The  strength  of  the  solution  will  slowly  vary,  so  that  it 
should  be  occasionally  titrated  against  the  permanganate,  which 
remains  unaltered,  to  determine  the  correct  equivalent  values. 
It  simplifies  the  calculations  if  the  two  solutions  are  diluted 
until  just  equivalent,  although  this  is  not  essential. 

Method  of  Operation. — Dissolve  I'l  grm.  in  a  conical  flask 
in  35  c.c.  of  1*2  sp.  gr.  nitric  acid.  Cool  and  dilute  to  about 
80  c.c.,  and  add  sodium  bismuthate  until  a  permanganate  colour 
persists.  Reduce  with  ferrous  ammonium  sulphate  and  cool. 
The  presence  of  chromium  is  indicated  by  a  brown  coloration 
which  must  be  destroyed  by  the  ferrous  ammonium  sulphate. 
To  the  cool  solution  add  a  slight  excess  of  sodium  bismuthate ; 
usually  about  1  grm.  is  sufficient.  Filter  through  asbestos, 
using  a  filter  pump,  and  wash  with  a  2  per  cent,  nitric  acid 
solution  until  the  filtrate  is  colourless.  Add  a  measured  excess 
of  ferrous  ammonium  sulphate  and  titrate  back  with  perman- 
ganate to  a  very  faint  coloration.  Each  c.c.  of  the  perman- 
ganate used  is  equivalent  to  *1  per  cent  Mn  in  the  steel,  if 
the  permanganate  solution  is  decinormal. 

Chromium  Determination. — Alloy  steels  containing  chrom- 
ium are  frequently  made  from  scrap  charges  containing  an 
appreciable  percentage  of  chromium,  which  it  may  not  be  de- 
sirable to  eliminate  under  a  strongly  oxidising  slag.  In  such 
cases,  when  an  addition  of  ferro-chrome  has  to  be  made  to 


336  THE    ELECTKO-METALLUKGY   OF    STEEL 

raise  the  chromium  to  the  specification  figure,  it  is  necessary  to 
take  a  bath  sample  for  analysis  at  a  time  when  slag  reactions, 
either  oxidising  or  reducing,  are  no  longer  capable  of  varying 
the  chromium  in  the  bath. 

Solutions  required. — (a)  Decinormal  Solution  of  Potassium 
Permanganate. — Dissolve  316  grms.  of  KMn04  and  make  up 
to  1000  c.c. 

(&)  Decinormal  Solution  of  Ferrous  Ammonium  Sulphate.— 
Dissolve  39 -6  grms.  of  ferrous  ammonium  sulphate  in  3  per 
cent,  sulphuric  acid  and  make  up  to  2000  c.c.,  using  the  same  acid. 
This  solution  is  decinormal  strength  and  should  be  equivalent 

N 
to  the  y^r  permanganate  solution. 

(c)  Solvent  Mixture  for  Steels. — Make  up  a  stock  solution 
consisting  of  the  following  :— 

300  c.c.  HN03. 

300  c.c.  H2S04  (one  part  in  three). 

300  c.c.  H20. 

1^  grms.  manganous  sulphate. 

Method  of  Operation  (N.  M.  Kandall). — Dissolve  1  grm. 
of  drillings  in  a  conical  flask  in  25  c.c.  of  the  solvent  solution, 
avoiding  evaporation.  When  solution  is  complete,  including 
all  light  particles  of  metallic  carbides,  add  20  c.c.  of  H20  and 
then  1  grm.  of  sodium  bismuthate.  Boil  for  three  minutes 
and  add  just  enough  HC1 — 1  or  2  c.c.  is  usually  sufficient 
—to  clear  the  solution  and  destroy  the  permanganate  colora- 
tion, after  which  boil  again  for  a  further  two  minutes. 

Dilute  with  200  c.c.  of  cold  water  and  add  a  measured  slight 
excess  of  ferrous  ammonium  sulphate.  Titrate  with  perman- 
ganate, which  gives  the  amount  of  ferrous  ammonium  sulphate 
added  in  excess  of  the  quantity  required  to  reduce  the  Cr03. 
This  amount,  deducted  from  the  total  quantity  added,  gives  the 
amount  oxidised,  which  multiplied  by  '001736  gives  the  weight 
of  chromium  in  solution. 

Nickel  Determination. — Nickel  steels  are  largely  made  from 
cold  charges  containing  nickel  steel  scrap.  In  such  cases  the 
charge  will  generally  contain  rather  less  nickel  than  is  required 
in  the  finished  steel,  so  that  it  is  necessary  to  know  the  percen- 
tage in  the  bath  before  a  correct  addition  of  nickel  can  be  made 


APPENDI X  337 

to  raise  it  to  the  specification  figure.     Nickel  is  not  easily  oxidised 
like  chromium  and  manganese  which  pass  into  the  slag,  so  that 
a  bath  sample  taken  whilst  the  slag  conditions  are  still  oxidising 
will  give  results  sufficiently  accurate. 

Solutions  Esquired. — (a)  Silver  Nitrate. — Dissolve  5*79 
grms.  recrystallised  AgN03  in  distilled  water  and  dilute  to  1000 
c.c. ;  1  c.c.  of  the  solution  is  then  equivalent  to  *001  grm.  of  nickel. 

(b)  Potassium  Cyanide. — Dissolve  5  grms.  purest  KCN  and 
5  grms.  KOH  in  water  and  dilute  to  1  litre. 

Titrate  this  solution  against  the  AgNO3  solution  and  dilute 
or  strengthen  until  just  equivalent ;  this  is  not  essential  but  it  is 
convenient  for  calculation  purposes. 

(c)  Ammonium   Sulphate. — Dissolve   400   grms.  (NH4)2S04 
and  make  up  to  1  litre. 

(d)  Ammonia. — Make  up  a  10  per  cent,  solution. 

(e)  Potassium  Iodide. — Make  up  a  2  per  cent,  solution. 
Method  of  Operation. — Weigh  1  grm.   of   drillings  into  a 

beaker  and  dissolve  in  20  c.c.  of  50  per  cent.  HC1. 

When  dissolved  add  5  or  6  c.c.  strong  HNO3.  Boil  to 
expel  nitrous  fumes  and  then  wash  into  a  500  c.c.  registered 
flask  and  dilute  to  about  120  c.c.  with  cold  water. 

Carefully  neutralise  with  10  per  cent,  ammonia,  shaking 
vigorously  between  each  addition ;  cool  and  add  a  measured 
quantity  of  KCN  solution,  using  10  c.c.  for  every  1  per  cent. 
Ni  expected  in  the  sample  plus  5  or  10  c.c.  in  excess.  Shake 
and  immediately  add  50  c.c.  of  10  per  cent,  ammonia,  make  up 
to  500  c.c.  and  pour  on  to  a  large  rapid  filtering  paper. 

Take  250  c.c.  of  the  filtrate,  add  6  c.c.  Am2SO4  solution  and 
2  c.c.  of  KI  solution.  Titrate  carefully  with  the  standard 
solution  of  AgN03  until  a  very  faint  permanent  opalescence  per- 
sists. Some  prefer  to  add  a  slight  excess  of  AgN03  and  then 
again  just  clear  with  the  KCN  solution. 

The  total  quantity  of  AgNO3  x  2  represents  the  excess  of  KCN 
added  to  the  original  steel  solution  before  making  it  up  to  500 
c.c.  and  halving  its  bulk 

The  quantity  of  KCN  consumed  in  the  formation  of  KNi(CN)2 
is  then  obtained  by  subtraction,  and  the  equivalent  c.c.  of 
AgNO3  solution  divided  by  10  gives  the  percentage  of  Ni  present. 

The   results   by  this   method   are   sufficiently  accurate   for 


338  THE    ELECTRO-METALLUKGY   OF    STEEL 

general  purposes,  but  sometimes,  in  order  to  arrive  at  a  more 
accurate  result,  the  actual  AgN03  used  in  titration  is  increased 
by  one-tenth  before  calculating  the  percentage  of  N;. 

A  nickel  determination  can  be  made  in  twenty  minutes. 

Tungsten  Determination. — Tungsten  behaves  similarly  to 
chromium  so  far  as  it  is  oxidised  under  an  oxidising  slag  and 
reduced  from  its  oxide  or  combined  oxides  dissolved  in  a  highly 
reducing  slag.  Therefore,  a  bath  sample  must  be  taken  at  a 
time  when  the  tungsten  is  no  longer  subject  to  variation,  or,  in 
other  words,  wrhen  the  slag  is  free  from  tungstic  oxides  and  has 
no  power  of  oxidation. 

Method  of  Operation. — Weigh  out  *5  grm.  of  drillings  or 
powder  crushed  in  a  percussion  mortar  and  fuse  in  a  large 
platinum  crucible  with  15  grms.  of  potassium  bisulphate.  Only 
one-third  part  of  the  bisulphate  is  at  first  used,  as  the  action  is 
violent.  Keep  the  crucible  well  covered,  and  heat  until  fumes 
are  evolved.  Remove  the  flame  for  a  minute  or  so,  allowing  the 
boiling  action  to  subside.  Allow  to  cool  somewhat,  add  another 
third  part  of  the  bisulphate  and  gradually  raise  to  redness. 
Repeat  until  all  the  bisulphate  is  fused.  Usually  fifteen  minutes 
is  sufficient  for  the  complete  fusion.  Allow  to  cool,  transfer  the 
melt  to  a  beaker  and  boil  with  water  until  dissolved,  keeping 
the  bulk  down  to  75  c.c.  Add  20  c.c.  strong  HC1 ;  boil  until 
the  W03  precipitated  becomes  pure  yellow,  and  leave  in  a  warm 
place  for-  half-an-hour.  Filter  through  a  small  filter,  washing 
the  precipitate  with  10  per  cent,  ammonium  nitrate.  Dissolve 
the  precipitate  on  the  filter  in  hot  dilute  ammonia,  allowing  the 
solution  to  run  into  a  weighed  platinum  dish  and  washing  the 
filter  with  ammonium  nitrate.  Evaporate  to  dryness  and  heat 
until  all  the  ammonium  salts  are  decomposed.  Treat  the  residue 
twice  with  HF  to  drive  off  any  silica  and  weigh  the  W03.  The 
filtrate  from  the  first  precipitate  of  WO3  will  contain  a  small 
amount  of  W  in  solution,  and  should  be  evaporated  with  HC1 
to  effect  precipitation.  The  W03  is  then  filtered  and  weighed 
up  with  the  first  precipitate  or  treated  separately,  as  above. 

The  weight  of  WO3  multiplied  by  '793  =  the  weight  of 
tungsten. 

Carbon  Determination. — Bath  Sampling. — When  it  is  re- 
quired to  know  the  carbon  content  of  a  bath  of  steel  certain 


APPENDIX  339 

precautions  must  first  be  taken  to  obtain  a  truly  representative 
sample.  A  bath  of  steel  after  carburising  will  not  be  perfectly 
homogeneous  until  it  is  moderately  hot  and  has  been  me- 
chanically agitated,  which  applies  more  particularly  to  medium 
and  high  carbon  steels.  The  sample  should  only  be  taken  when 
the  bath  is  covered  by  well  fused  slag  low  in  metallic  oxides  and 
incapable  of  exerting  any  oxidation  of  the  carbon  in  the  steel ; 
a  crude  heat  test  should  also  be  taken  to  make  sure  that  the  steel 
is  not  cold  and  this  should  be  followed  by  a  vigorous  rabbling 
with  a  slag-covered  skimming  tool.  Unless  these  precautions 
are  taken  the  bath  sample  analysis  will  give  vitiated  results  and 
the  calculated  addition  of  carbon  subsequently  made  in  the  form 
of  ferro-alloys  or  pig  will  probably  produce  a  finished  steel  with 
a  carbon  content  outside  the  specification  limits. 

In  many  works  it  is  customary  to  use  the  colour  method  of 
carbon  determination,  which  is  quite  satisfactory  where  no  great 
degree  of  accuracy  is  required  and  when  the  carbon  is  moderately 
low.  The  colour  method  is  not  suitable  for  alloy  steels  or  for 
medium  and  high  carbon  samples,  owing  to  the  comparison  of 
the  colour  tints  being  vitiated  either  by  the  alloying  metals  or  by 
the  rate  at  which  the  sample  is  cooled.  Whenever  the  colour 
method  is  adopted,  the  standard  samples  chosen  should  resemble 
as  far  as  possible  the  bath  samples,  both  as  regards  analysis  and 
process  of  manufacture  ;  it  would,  for  example,  be  unwise  to  use 
a  highly  oxidised  steel  as  a  standard  for  comparison  with  a 
sample  of  "  killed  "  steel. 

The  direct  combustion  method  of  analysis  is  almost  invari- 
ably used  for  the  production  of  alloy  or  plain  carbon  steel  ingots 
to  a  close  specification. 

Principle  of  Method. — The  sample  of  steel  is  completely  burnt 
in  a  stream  of  oxygen,  oxidation  being  assisted  by  covering  the 
sample  in  the  combustion  boat  with  a  small  quantity  of  either 
lead  chromate,  red  lead,  or  manganese  tetroxide.  These  com- 
pounds may  themselves  contain  traces  of  carbon  which  must  be 
carefully  determined  beforehand  and  allowed  for  in  every  sample 
of  steel  analysed. 

The  carbon  in  the  steel  is  burnt  almost  entirely  to  C02,  any 
CO  that  may  be  formed  being  oxidised  to  CO2  by  red  hot  cupric 
oxide  before  leaving  the  combustion  tube. 


340  THE    ELECTRO-METALLURGY   OF    STEEL 

The  C02  is  slowly  swept  out  of  the  tube  by  a  stream  of 
oxygen  and  air,  and  is  purified  before  reaching  the  absorp- 
tion tube.  The  C02  is  best  absorbed  by  soda  lime  for  rapid 
working. 

Preparation  of  Purifying  Absorbents. — (a)  Caustic  Potash 
Solution. — This  solution  is  made  up  to  50  per  cent,  strength 
and  is  used  for  removing  any  CO2  from  the  oxygen  or  air  before 
entering  the  combustion  tube. 

(b)  Cupric  Oxide  Gauze. — A  long  strip  of  fine  meshed  copper 
gauze  about  3  or  4  inches  wide  is  rolled  up  tightly  to  form  a 
plug,  which  can  be  pushed  into  the  combustion  tube.     This  plug 
occupies  a  position  towards  the  outlet  end  of  the  tube,  where 
it  will  be  raised  to  a  bright  red  heat.     The  copper  gauze  is  later 
converted  into  oxide  by  heating  to  a  high  temperature  in  the 
tube  through  which  a  stream  of  air  or  oxygen  is  passed. 

(c)  Chromic  Acid  Solution. — A  saturated  solution  of  chromium 
trioxide  in  water  is  generally  used  but  may  be  substituted  by  a 
saturated  solution  of  chromium  trioxide  in  dilute  H2S04. 

(d)  Pure  Sulphuric  Acid. 

(e)  Anhydrous    Fused    Calcium   Chloride.  —  The   prepared 
chloride  is  crushed  to  the  size  of  small  peas,  freed  from  dust, 
and  dried  at  200°  C.  before  filling  into  the  U  tubes. 

(/)  Soda  Lime. — Soda  lime  may  be  prepared  by  dissolving 
CaO  and  NaOH  in  their  molecular  proportions  in  water,  and 
evaporating  the  solution  to  dryness.  The  dried  residue  is 
broken  up  and  crushed  to  pass  a  10-mesh  sieve  and  is  slightly 
damped  with  water,  containing  a  few  drops  of  a  5  per  cent, 
phenol-phthalein  solution  in  alcohol,  before  packing  into  the 
absorption  tube. 

Apparatus  and  Method  of  Determination. — The  apparatus, 
consisting  of  gas  washing  bottles,  electric  combustion  furnace, 
SO2  absorbing  bulbs,  drying  bulbs,  002  absorption  U  tube  and 
aspirator,  is  shown  assembled  in  Fig.  132. 

The  two  gas  washing  bottles  A  and  B,  shown  to  the  right- 
hand  side  of  the  apparatus,  are  filled  with  strong  sulphuric 
acid  and  with  KOH  solution  respectively,  the  latter  being 
nearest  the  oxygen  cylinder.  Various  types  of  washing  bottles 
or  absorption  bulbs  may  be  used  in  place  of  the  Wolff's  bottles 
shown.  The  H2S04  bottle  is  attached  by  a  short  length  of  pres- 


APPENDIX  341 

sure  tubing  to  a  rubber  cork,  which  fits  tightly  into  the  com- 
bustion tube  C. 

The  combustion  tube  may  be  either  porcelain  or  fused  silica 
about  1  inch  internal  diameter  or  large  enough  to  take  the  com- 
bustion boat,  which  is  usually  fire-clay  or  porcelain.  Fire-clay 
boats  should  be  strongly  ignited  in  a  muffle  before  use  to  ensure 
perfect  freedom  from  carbon.  The  outlet  end  of  the  combustion 
tube  is  connected  to  an  empty  bulb  D  which  removes  the  bulk 
of  the  moisture  carried  over  by  simple  condensation.  The  next 
bulb  E  contains  the  chromic  acid  solution  which  oxidises  any 
S02  passing  over  with  the  gas  to  SO3,  which  is  then  absorbed. 
The  bulb  F  contains  strong  H2SO4  which  further  dries  the  gas 
before  it  passes  through  the  U  tube  G  containing  anhydrous 
calcium  chloride  which  abstracts  the  last  traces  of  water  vapour. 
The  U  tube  H  is  the  weighed  absorption  tube,  and  is  packed 
with  soda  lime  in  the  right-hand  limb  and  with  calcium  chloride 
in  the  other.  The  calcium  chloride  in  this  U  tube  absorbs  any 
moisture  which  might  be  driven  off  from  the  soda  lime.  Lastly, 
a  U  tube  I  filled  with  calcium  chloride  completes  the  combustion 
train,  and  prevents  the  weighed  absorption  U  tube  H  from  ab- 
sorbing any  moisture  from  the  atmosphere.  The  Winchester 
quart  bottles  shown  serve  as  a  convenient  form  of  aspirator  and 
are  so  arranged  that  water  syphons  over  from  the  upper  bottle 
into  the  lower  one,  drawing  either  oxygen  or  air  through  the 
apparatus. 

Before  using  a  newly  charged  weighing  tube  for  an  actual 
carbon  determination  it  should  be  connected  up  in  the  train 
through  which  air  is  then  passed,  the  furnace  being  at  a  full 
heat  to  resemble  actual  working  conditions.  After  passing  .air 
for  about  fifteen  minutes,  the  weighing  tube  is  disconnected  and 
carefully  weighed ;  it  should  then  be  again  connected  in  the 
train,  and  the  process  repeated  until  its  weight  is  constant. 
The  apparatus  is  then  ready  for  .use. 

Two  grms.  of  fine  drillings  or  powder  (in  the  case  of  self- 
hardening  steels)  are  mixed  with  2  grms.  of  red  lead  and  placed 
in  the  combustion  boat,  which  is  then  quickly  pushed  into  the 
hottest  part  of  the  combustion  tube ;  the  latter  should  be  at  a 
temperature  of  at  least  800°  C.  The  washing  bottles  A  and  B 
are  then  connected  to  the  oxygen  cylinder  or  a  gas  container 


342  THE   ELECTEO-METALLTJEGY   OF   STEEL 

and  to  the  combustion  tube.  The  pressure  in  the  train  is  then 
reduced  by  coupling  up  the  aspirator  so  that  the  water  endeavours 
to  syphon  over  from  the  upper  bottle.  In  about  three  minutes 
the  oxygen  is  turned  on  and  a  stream  of  gas  passed  through  the 
apparatus  so  that  the  Winchester  bottle  is  emptied  in  about  five 
minutes. 

The  oxygen  is  then  shut  off,  the  aspirator  bottles  reversed, 
and  a  stream  of  air  drawn  through  until  the  upper  bottle  is 
again  emptied.  The  weighing  U  tube  H  is  then  disconnected 
and  weighed,  the  increase  of  weight  being  due  to  the  C02  absorbed. 
The  phenol-phthalein  loses  its  pink  colour  when  the  soda  lime 
becomes  exhausted,  and  requires  renewal. 

A  carbon  determination  by  this  method  should  be  completed 
within  twenty  minutes  of  receiving  the  prepared  sample. 


INDEX. 


ACID  process  of  liquid  refining —  PAGES 

Application  and  scope  of         .        .         .        ...        .        .          163,  164 

General  description  of     .        .        .        .        .        .';...  159-163 

Slag  used  in    .         .        .        ...        .        .        .        .        .        .        161 

Theories  of .  .          159-163 

Acid  process  of  melting  and  refining — 

Application  and  scope  of          .         .         .        .        ,        .  114,115,150 

to  foundry  practice      .        .        .        .        .        .        .    •      198-200 

Carbon  removal  by  slag  reaction  in        .        ...        .        .  153-155 

Ferro-alloy  additions,  calculation  of  .        .        .        .        .        .         156 

General  outline  of         .".'.' 150-152 

Scrap,  choice  of       ...........         152 

—  ,  method  of  charging      .        .        *        ...        .        .         .         152 

Slag,  analysis  of      .         .         .        .;•'.' 156 

—  ,  formation  of 153,  154 

—  ,  function  of 153,  154 

—  ,  physical  and  chemical  characteristics  of        .        .        .        .        .        155 
Alternating  current,  definition  of 42 

—        supply,  definition  of 42 

Alternators,  single-phase        .         .        .        .        .  • 75 

Alundun         .         .         .      ^ .'        .        .        . 287 

Ammeters,  use  of  .         .        .        i 96 

Analysis,  rapid  methods  of  332-342 

Carbon,  determination  of 339-342 

—  Chromium,  335-336 

—  Manganese,  334-335 

Nickel,  .  ' 336-338 

—  Phosphorus,        —  332-334 

—  Tungsten,  338 

Annealing  castings 207-209 

Arc  furnaces,  direct,  see  "  Direct  arc  furnaces  ". 

—      ,  indirect,  see  "  Indirect  arc  furnaces  ". 

— ,  high  voltage    .        . 222 

—  ,  low  voltage      .        .        *        .        .        .        .        .        .        .        .        .        221 

BASIC  process  of  liquid  refining — 

Application  and  scope  of .  163,  164 

General  description  of .  .        .  157-159 

Basic  process  of  melting  and  refining — 

Application  and  scope  of .  113-115 

to  foundry  practice .  199 

Carbon  removal  by  oxidising  slag  in       ....  .  125 

Carburising,  calculations  for .  141-147 

,  method  of 131 

Deoxidation  of  bath .  136-140 

Ferro-alloy  additions,  calculation  of       ....  .  139-147 

Fluxes  used  in 122,  133,  134 

General  outline  of .  115-117 

Phosphorus,  removal  of .  126,  127 

Power  consumption  in 112 

(343) 


344 


THE   ELECTRO-METALLURGY   OF    STEEL 


Basic  process  of  melting  and  refining  (cont.) —  PAGES 

Scrap,  choice  of       .         .         .         .         : 117-123 

—    ,  method  of  charging 120-122 

Skimming,  condition  of  bath  before 128-130 

,  method  of 130 

Slag,  function  of  oxidising 127 

-  ,  reducing 138,  139 

Temperature,  control  of 130,  140 

,  methods  of  testing    .        .         .         .    ^    .        .        .         .         140 

Bassanese  furnace 40 

Booth-Hall  furnace — 

Characteristic  features  of 275 

Construction  of 276 

Electrical  design  of 41,  59,  275 

Hearth,  construction  of 278 

Lining  of 278 

Operation  and  manipulation  of 279 

Power  rating  of 276 

Bricks,  bauxite 286 

—  ,  cheek 194,  195 

—  ,  dozzle         , 194 

-  ,  magnesite 284 

—  ,  silica 287  290 

-  ,  trumpet 192 

CARBON,  determination  of,  in  steel 339-342 

,  oxidation  of,  by  acid  slags 153-155 

,  ,  by  basic  slags 125 

Carburising,  calculations  for 141-147 

—        ,  method  of 131 

Castings,  electric  steel- 
Annealing  of 207-209 

Blowholes  in 210 

Brittleness 231 

Defects  of 210-213 

Gas  cavities  in 211 

Mechanical  tests  of 206-208 

Shrinkage  cavities  in 212 

Slag,  inclusions  in 211,  212 

Specifications  for 205,  206 

Tears,  cracks,  and  pulls 212,  213 

Cheek  bricks 194,  195 

Chromite 287 

Chromium  in  steel,  determination  of 335,  336 

Colby  furnace 4-6 

Conductive  hearth  furnaces — 

Advantages  and  disadvantages  of 225,  226 

Booth-Hall  type  of 41,  275-279 

Dixon  type  of 40 

Early  use  of  basic  bottoms  for 37 

Electro-metals  type  of 38,  55,  71-73,  257-259 

Girod  embedded  electrode  type  of 33-36,  56,  57,  252-255 

Greaves-Etchells  type  of 41,  270-274 

Keller  composite  bottom  type  of 31,  32 

Stobie  type  of 38,  260-263 

Conductive  hearths  of  furnaces — 

Advantages  and  disadvantages  of 225-226 

Construction  of 296-299 

Costs,  power  consumption 112 

Current,  effective,  definition  of 44 

—  ,  fluctuation 49,  111,  112 

—  ,  transformers 97 

Cycle,  definition  of  43 


INDEX  345 

PAGES 

DEOXIDATION  of  steel,  by  basic  reducing  slag 136-138 

,  Girod's  patents  for .  36 

,  Heroult's  patents  for 22,  24,  25 

Design  of  arc  furnaces — 

Chemical  reactions  and  their  influence  upon — 

Corrosive  action  of  basic  fumes .  214 

oxide  fumes '  .         .  216 

—        slag   .        .      . 216 

Erosion  of  bottom*.         . 215 

Lime  dust 215 

Maintenance  of  definite  conditions 216 

Slag,  circulation  of 215 

—  ,  skimming  of 215 

Electrical  considerations  in — 

Conductors .'  .  222-224 

Heat  conversion  of  electric  energy           .  .        .'  -  .  .  225-226 

High  voltage  arcs     .        .        .        .        .  '.        .  ,.-•.'-  ..  .        222 

Low  voltage  arcs      .                 .        .        .        .        .  ...  *        221 

Power  factor    .        .        .        .        . '      .        ...  .  .  228,  229 

Transformer  capacity      .       ..        .        .        .       .,  ,.  .  .        224 

Mechanical  features  of — 

Electrode  holders    .        .        .        .        .        .-'•>  ;    .  .  .  .        232 

—  regulating  gears       .        .        ..        .  '-.-.-•  .  .  230,231 
Furnace  doors .  '  -.  .  .         233 

—  mountings         *   '     .       ..        .;       .    ''•»'.      .    .    .         229,230 
Tilting  gears    .         .        ...         .        .         .        .        .         .        230 

Physical  conditions  and  their  influence  upon — 

Heat  distribution     .....       ..       ...  ...  .        217 

—  loss  by  radiation     .        .        .        .       . .  .        .        .  218,  219 

Intensity  of  arc  heating   ....       ..        .        .  .        .        .  216,217 

Temperature  variation    .        .        ...  .        .        .  .        217 

Direct  arc  furnaces- 
See    Booth-Hall,     Dixon,    Electro-metals,     Giffre,    Girod,    Greaves- 
Etchells,  Heroult,  Keller,  Ludlum,  Siemens,  and  Stobie  furnaces. 

Dixon  furnace — 

Four-phase  type     .        ...        .        ...        .        .        .         .70,71 

Patents  .         .        ......'        .  •.        .         40,  58,  70, 71 

Two-phase  type .....        .         .         .          58 

Dolomite         .         .         .....        .        .        .        .         .         .         .        283 

Dozzles  .        .        .      • .        .  .        .        . 194 

Duplex  processes — 

Girod's  patents 25 

Methods  of  applying       .        .        .        .        .        „        .        .         .          163-164 

EFFECTIVE  current,  definition  of 44 

—         voltage,  „ 44 

Electrical  degree,  definition  of 43 

Electrodes — 

Amorphous,  chemical  and  physical  characteristics  of   .        .        .          312-315 

—  ,  comparison  with  graphite  electrodes          .         .        .        .        321 

—  ,  current  carrying  capacity  of 320 

—  .defects  of 310-312 

—  ,  economises  for 324-329 

—  ,  joining  of 317-319 

—  ,  manufacture  of 308-310 

—  ,  materials  used  in  manufacture  of        ....          306-307 

—  ,  storage  of 320 

Graphite,  artificial 307,  308,  316-319 

,  comparison  with  amorphous  electrodes          ....        321 

—  ,  current  carrying  capacity  of 320 

—  ,  economisers  for  324-329 


346  THE   ELECTRO-METALLURGY  OF   STEEL 

Electrodes  (cont.) —  PAGES 

Graphite,  joining  of 319 

—  ,  manufacture  of 316 

—  ,  natural 307 

Holders  for,  design  of 232 

Regulating  gear  for         .         . 230,  231 

Electro-metals  furnace — 

Constructional  features  of 257 

Electrical  features  of 38,  55,  71-73 

Electrodes  used  in           ......."....  259 

Four-phase  type 71-73 

Linings  of 258 

Operation  and  manipulation  of 259 

Patents 38 

FERRANTI  furnace 3,  4 

Ferro-alloy  additions  to  liquid  steel — 

In  acid  process,  calculation  of 156 

In  basic      „  „  139-147 

Fettling,  method  of 148,  149 

Fire-clay 290,  291 

Flat  rate  system,  purchase  of  power  on 103 

Fluctuation  of  load,  effect  on  load  factor Ill,  112 

Fluxes,  used  in  basic  process 122,  133,  134 

Foundry  practice — 

Application  of  acid  process  to 198-200 

basic        „  199 

Electric  melting  in,  advantages  and  disadvantages  of     .         .         .  197-202 

Furnace  capacity,  choice  of 204,  205 

Lay-out  of  furnace  plant 203,  204 

Statistics  of  castings  production 200,  202,  203 

Four-phase  circuits,  electro-metals,  grouping  of 71 

—  ,  mesh  connection  of 71 

—  ,  star  connection  of 70 

current,  application  of 69-73 

furnaces,  Dixon  types 70-71 

—  ,  Electro-metals  type 71-73 

Frequency,  definition  of  42 

Frick  furnace 13,  14 

Furnace  capacity,  in  relation  to  foundry  practice        .         .         .         .         204,  205 

—       ,                           radiation  loss     .         .         .         .         .         218,  219 
Furnace  linings- 
Acid         303 

Baking  in  basic  hearths 299-303 

Basic,  with  bottom  electrodes 225,  226,  296-299 

-  ,  without  bottom  electrodes 292-296 

Drying-out 299-303 

Life  of 305 

Repairs  to 304,  305 

Roof  construction 299 

GANISTEB 289,  290 

Giffre  furnace,  single-phase  type 52 

•     ,  three-phase  type 67 

Girod  furnace — 

Bethlehem  Steel  Co.,  U.S.A.,  description  of 254 

Bottom  electrode  losses  in 255 

Constructional  features  of 254,  255 

Early  types  of 33-35 

Electrical  features  of 56,  67,  252-254 

Linings  of 255 

Patents  for  .     33-36 


INDEX  347 

Girod  furnace  (cont.) —  PAGES 

Single-phase  type 252-253 

Two-phase  type 56 

Three  phase  type 67,  254 

Greaves-Etchells  furnace — 

Constructional  features  of 273 

Electrical  features  of 68,  270-272 

Hearth  construction 274 

—  resistance 273 

Lining  of 274 

Patents  for 41 

HEAT — 

Distribution  of,  in  arc  furnaces 217 

Radiation,  loss  of 218,  219 

—  —        ,  in  relation  to  furnace  design 218 

—  —        ,  thermal  efficiency       .        .         .          108-109 
Heroult  furnace — 

Construction  of  modern  types  of 247,  249,  250,  252 

Development  of 244 

Early  types  and  history  of 20,  21 

Electrical  features  of  modern  types .66,  245-247 

Electrodes  used  in 252 

Linings  of  .        .        .        . 252 

Patents  for 21-25 

Power  factor  of 246 

Single-phase  type 20,  25,  55 

Two-phase  type 55 

Hiorth  furnace 15,  16 

INDIRECT  arc  furnaces— 

Bassanese  furnace 40 

Rennerfelt  furnace 39,  237-244 

Siemens  furnace 2 

Snyder  furnace 39,  263-270 

Stassano,  ore  smelting  furnace 17-19 

—  ,  steel  melting  furnace 19,  234-237 

Induction  and  induced  currents,  definition  of 45 

Induction  furnaces — 

Colby  furnace 4-6 

Ferranti  furnace ,         3-4 

Frick  furnace 13-14 

Hiorth  furnace 15,  16 

Kjellin  furnace 6-9 

Rochling-Rodenhauser  furnace .        .       9-13 

Ingots — 

Bottom  casting  of 190-194 

Defects  of 171-181 

Blowholes 180 

Clinks 178 

Contraction  cavities         .        .        .        .        .        .        .        .        .         178 

Folding  and  lapping        .        .        .        .        .    "   .•       .        .         175,176 

Occluded  gases         .        .        .         .        .        .        .        .        .         180,  181 

Pipes 171-173 

Pulls         .         .         .         .        .        .        ...        .        .        .         177 

Segregation      .......'...          173-175 

Shell .--...        .         176,177 

Surface  cracks          .        .        .        .  .        .        .        *.-••      178-180 

—       pitting        .         .        .        .,       .        .  -  .-.        .        .        .         177 

Theory  of  formation        .         .         ...         .        .         .        .          165-171 

Top-casting  of         .        .        .        '.-       .        i        .        .        ...          188-190 

Tun-dish  casting  of        .        .        .        .        .        *        .        .        .         195,196 


348  THE   ELECTBO-METALLUBGY   OF   STEEL. 

PAGES 

Ingot  moulds,  grey  iron  for  casting       .........         186 

,  influence  of  shape  on  ingot  formation   ....          186-190 

P^ 188 

KELLER  furnace — 

Conductive  hearth  type 31,  32 

Early  single-phase  type 26 

Patents  for 28-31 

Single  phase,  four  electrode 26-28 

Three-phase 67 

Kjellin  furnace 6-9 

LADLES,  bottom  teeming — 

Heating  appliances  for 185,  186 

Use  and  manipulation  of 181-185 

Lag,  definition  of 45 

Lead,  definition  of 45 

Linings,  see  "  Furnace  linings  ". 

Liquid  refining,  Heroult's  patents  for  .    ' 25 

See  also  "  Acid  process  of  "  and  "  Basic  process  of ". 

Load  factor — 

Definition  of 51 

Effect  of  fluctuating  current  upon Ill,  112 

—  ,  on  cost  of  power 105-107 

—  ,  on  energy  consumption 110,  111 

,  on  output 110,  111 

Maximum  demand,  in  relation  to 107 

Ludlum  furnace .         .         .        280 

MAGNESITE 284 

Manganese,  determination  of,  in  steel 334,  335 

,  removal  of,  by  oxidising  slag 127 

Maximum  demand — 

Contract  for  power  based  upon 104,  105 

Definition  of 105 

Eelation  of,  to  cost  of  power 108 

—        ,  to  load  factor 107 

Meter  rate  system,  purchase  of  power  on 103 

NICKEL,  determination  of,  in  steel         .        .        .        .  .        .          336-338 

PERIOD,  definition  of •  43 

Phase,  definition  of 43 

Phosphorus,  determination  of,  in  steel 333-334 

,  removal  of,  in  basic  process 126-127 

Power — 

Consumption  of  (energy),  for  melting  and  refining        ....         112 

—       ,  influence  of  load  factor  upon          .         .         .         112 

,  power  input  upon       .         .         .         109 

Contracts  for,  basis  of 102,  103 

—  ,  on  "  flat  rate  "  system 103 

—  ,  on  "  maximum  demand  "  system     ....         104,105 

—  ,  on  "  meter-rate  "  system 103 

Cost  of,  effect  of  load  factor  upon 105-107 

—    ,        —      maximum  demand  upon 108 

Power  factor — 

Definition  of   .         .  - 48 

Heroult  furnace,  power  factor  of 228,  229 

Relation  of,  to  furnace  design 84,  85 

,  to  reactance  voltage 266 

Snyder  furnace,  power  factor  of 246 


INDEX  349 

RADIATION,  heat  loss  by—  PAGES 

Relation  of,  to  furnace  design 218,  219 

—        ,  to  load  input 108,  109 

—  ,  to  useful  energy  input 109 

Reactance — 

Coils 83-85 

—  ,  voltage  variation  by  means  of 83 

Definition  of 47 

Drop,  definition  of 48 

Internal,  of  transformers 77 

Voltage 48,  84 

Refractory  materials — 

Acid,  analysis  of 290 

Alundum 287 

Bauxite 286 

Chromite 287 

Classification  of 281 

Conditions  of  service  of,  in  electric  furnaces          ....          281-283 

Dolomite 283 

Fire-clay 290-291 

Ganister .        ...         289,  290 

Magnesite ......        284 

Silica       . .,       .          287-290 

Regulation — 

Automatic,  applied  to  polyphase  arc  furnaces 88,  89 

—  ,        —         single -phase  constant  pressure  circuits        .         .          88 

—  ,        —  direct  arc  furnaces  .        .        22,  86-88 

—  ,        —  indirect  arc  furnaces       .        ;        .          86 

—  ,  principles  of          .        ..'.      .    "    .        ,'      .    *    .        .        .     86-89 
Regulators — 

Combined  current  and  voltage,  General  Electric  Co.,  U.S.A.,  type        ..-•       95 
—     ,  Watford  type        .....          95 

Thury,  description  of..        .        .        .        .        .        .        .        .90 

—     ,  diagram  of  connections      .        .        .  ......        ~          94 

Voltage,  Watford  type    ..."       .-        .        .        ...        .        .93,94 

Rennerfelt  furnace — 

Constructional  features  of  .        .  ."  .        240 

Electrical  features  of      .        ....        .        .        .        ...    57,237-240 

Electrodes  used  in  .        .         .         .        ...        .        .        .         241,242 

,  consumption  of        • 243 

Linings  of .         240,  241 

Operation  of .        .        .        .        243 

Patents  for     .         .         .        .        . ,  .        .39 

Rochling-Rodenhauser  furnace     ......        ,        .        .        .        .       9-13 

Roofs  of  furnaces  « 299 

SAMPLES,  methods  of  taking  .  .        .        .  .        §        .  -      330, 338,  339 

—  ,  preparation  of        .  .  ...  .        .        <        .        331 

Scrap- 
Choice  of,  for  acid  process  .  .        .         .  .        ..»        .        152 

—  ,  for  basic  process    .        .        .  .                 .        .        .          117-123 

—  ,  method  of  charging        .        .  .        .    -  .        .           120-122,  152 

Self-induction,  definition  of  .         .         .  .        .        ...        .          47 

Siemens'  direct  arc  furnace  ..'._. 2 

—  indirect  arc  furnace        .        .        .  .        .        .        «        . 

Silica  bricks,  cement  and  sand  .        .        f        .        .      .  .  .        •*          287-290 
Single  phase- 
Current,  applications  of  .        ..-.        .        .  .        .        ..       52 

—  ,  definition  of. ..'.          50 

—  generators         .        .        .        .        .        ...        .        .          75 

,  Miles- Walker  type  of    .       ...       .        .      .  .  76 

—  ,  supply  of       •     t        .        M        .        •        •        •        •        •        .  74 


350  THE   ELECTEO-METALLUEGY   OF    STEEL 

Single-phase  (cont.) —  PAGES 

Furnaces  (arc  type) — 

See  Bassanese,  Girod,    Heroult,  Keller,    Siemens,   Snyder,   and 

Stassano  furnaces. 
Furnaces  (induction  type) — 

See    Colby,    Ferranti,    Frick,    Hiorth,   Kjellin,    and    Rochling- 
Rodenhauser  furnaces. 

Transformers  (static) 77 

Sink-head 195 

Slag- 
Acid,  analysis  of .         156 

—  ,  characteristics  of 155 

—  ,  formation  of 153,  154 

—  ,  function  of 153,  154 

Basic  oxidising,  character  of 123 

—  ,  composition  of 123 

,  fluxes  for 122 

—  —       ,  formation  of 124 

—  ,  function  of 122 

—  —       ,  reactions  of 125 

—  ,  removal  of  carbon  by 125,  126 

—  ,          —         manganese  and  silicon  by  .         .         .         .         127 

—  ,  phosphorus  by 126,  127 

—  —       ,  sulphur  by 127 

—  reducing,  analysis  of 135 

,  character  of    .         . 135,  136 

—  ,  deoxidising  powers  of 136-138 

,  fluxes  used  for 133,  134 

—  —      ,  formation  of 136 

—  —      ,  function  of 132,  133 

—  ,  reactions  of 138,  139 

Slag  inclusions 211,  212 

Snyder  furnace — 

Constructional  features  of 267  269 

Electrical  features  of .          263-267 

Lining  of 269 

Load  input,  current  and  voltage  curves  of 263-265 

Operation  and  manipulation  of 269,  270 

Patents  for 39,  40 

Power  factor  of 266 

Specifications,  for  castings 205,  206 

Stassano  furnace — 

Ore  smelting  types 17-19 

Steel  melting  types,  constructional  features  of       ....          234,  235 

—  ,  electrical  features  of 66,  236,  237 

—  ,  patents  for 19 

Stobie  furnace — 

Constructional  features  of 260-262 

Electrical  features  of 57,  68,  260 

Electrode  economisers  of 262 

Electrodes  used  in 262,  263 

Lining  of 262 

Patents  for 38 

Sulphur — 

Removal  from  steel,  by  Heroult's  process 22,  24,  25 

,  by  oxidising  basic  slags 127 

,  by  reducing      „  ....          138,  139 

Switch  gear,  change  voltage          .........          81 

-  ,  high  tension 99-101 

TEMPERATURE,  control  of 130,  140 

,  methods  of  testing  bath 140 


INDEX  351 

Three-phase—  PAGES 

Circuits,  "  delta  "  or  "  mesh  "  connection  of 62-64 

,  four  wire  star  connection  of 67,  68 

—  ,  inverted  star  connection  of 65 

—  ,  nomenclature  of .  62 

,  three-wire  star  connection  of 60 

Current,  application*  of 59-69 

—  ,  definition  of              .        .        .        .    - 50 

Furnaces  (arc  type) — 

See  Giffre,   Girod,   Greaves-Etchells,   Heroult,   Keller,   Ludlura, 

and  Stobie  furnaces. 
Furnaces  (induction  type) — 

See  Rochling-Rodenhauser  furnace. 

Tilting  gears 230 

Tools,  furnace 148 

Transformers  (static) — 

Capacity  of,  in  relation  to  furnace  capacity   .        .        .        .        .    ,   .  224 

—  ,       —        —     load  factor     .        .        .        .       ,.        .      -'..  224 

Current '   ...      .        .        .        .        .        .        .  97 

Grouping  of,  three-phase  to  three-phase          .        .        ...        .  79 

,                      to  two-phase    .         .        .                 .        ...  80 

—  ,  two -phase  to  three-phase  .        .         .        .        .        .  80 

Heating  of                .        ;        .        .        .        .        ....        .  77 

Internal  reactance  of      .     -  ..        .        .        .        .       ..       ...      .  ...  77 

Primary  tappings  of        .         ....        .        .        .        .        .        .  78 

Secondary  voltage,  variation  of       .        .        .        .     '  .        .       . .        .  80 

Single-phase    .         .        ...        .        .        .        .        .        .        .  77 

Trumpet  bricks      .        .        ...        .         .         .                 .        .         .  192 

Tun-dish .    ,    .        .        ....        .  196 

Tungsten,  determination  of,  in  steel      .        .        .         .        ...        .  338 

Two-phase — 

Current,  application  of  .                          .        .        .        .        .        .        .  53-59 

—  ,  definition  of      .        .        .        .        ..        .        .        .        .        .  50 

Furnaces — 

See  Booth-Hall,  Dixon,  Electro-metals,  Girod,  Heroult,  Eennerfelt, 
and  Stobie  furnaces. 

VOLTAGE,  circuit  or  line 44 

—  ,  effective       '  . 62,84 

—  ,  reactive          .        .-       . 48,  84 

—  ,  resistance 84 

Volt-ampere  and  K.V.A.,  definition  of  .        .        . ' 48 

WATT- and  K.W.,  definition  of       .        .        .        ;        .        .        .        .        .  49 

Wattless  current    .         .        .        .        .                 .         .                 .         .         .  48 

Watt-hour  meter,  use  of        .        .        .        -.    *    .        .        .        .        .        .  99 

Wattmeter- 
Graphic  recording,  use  of        .        .        .        .       • .  -     .        .        .        .  98 

Indicating,  use  of 98 

Waveform                      42 


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STAMPED  BELOW 

Books  not. returned  on  time  are  subject  to  a  fine  of 
50c  per  volume  after  the  third  day  overdue,  increasing 
to  $1.00  per  volume  after  the  sixth  day.  Books  not  in 
demand  may  be  renewed  if  application  is  made  before 
expiration  of  loan  period. 


'*!* 


MAR 


-3  23  K 


lOm-4,'23 


YC   18731 


500142 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


